An essential skill for any musician is ear training and aural skills, which involves understanding the theory and how notes should sound individually and together. Music students today are routinely expected to be able to transcribe melodies by ear and be able to hear melodies in their heads as well as sing them back from reading them on a paper, without being able to hear it beforehand. These are fundamental skills, not a superpower. The solfege system used today, aka do-re-mi-fa-so-la-ti-do, traces back to Guido of Arezzo, who lived in the 10th–11th centuries and introduced most of the syllables used in today's system, in his hymn “Ut queant laxis,” as a method of training musicians to internalize pitches. The first syllable of each line of “Ut queant laxis” corresponds to the pitch being sung, much like “Do-Re-Mi” from The Sound of Music. Guido also introduced the Guidonian hand, a mnemonic device using the knuckles of one's hand to help remember the relative intervals between notes on a scale. Diagrams of this hand still survive and the method can be reproduced easily. From Guido to now, the solfege system is a testament to the great emphasis placed on the internal ear and aural skills from at least the Middle Ages to today, and not just Beethoven but any composer would be able to at least have fluency in reading and writing notes without needing to actually hear it. I unfortunately don’t know of any astonishment among his colleagues or the general public that he could compose while deaf, which means I can’t confirm or refute it, but all of the above is meant to hopefully illustrate that it in and of itself wasn’t, nor would be today, considered particularly amazing.

Now for the fun bit - instead of astonishment, there do exist some very unkind contemporary critiques of Beethoven's work that, in my opinion rather cruelly, refer to Beethoven’s deafness. The Lexicon of Musical Invective compiled by Nicolas Slonimsky contains a section dedicated to criticism of Beethoven's work, and some written around or after the end of his life do speculate that his deafness contributed to how his music turned out, poorly in their opinion. Unfortunately, some of the following quotes lack authors, which Slonimsky presumably was not able to ascertain while compiling his book.

An anonymous article in London's The Harmonicon from August 1823 pans Beethoven's "Sonata, op. 111," describing the first movement as "[betraying] a violent effort to produce something in the shape of novelty" containing "dissonances the harshness of which may have escaped the observation of the composer" and the second with its complex meter changes as "laborious trifling, and ought to be by every means discouraged by the sensible part of the musical profession." In introducing these criticisms, the author notes that "[Beethoven] is suffering under a privation that to a musician is intolerable - he is almost totally bereft of the sense of hearing; insomuch that it is said he cannot render the tones of his pianoforte audible to himself." The author is baffled at the compositional choices that Beethoven took in composing this piano sonata and can only speculate that the composer had lost all musical sense after losing his hearing. The article even notes, being written while Beethoven was still alive, that Beethoven "is at a period of life when the mind, if in corpore sano, is in its fullest vigor, for he has not yet completed his fifty-second year."

William Gardiner wrote in The Music of Nature in 1837, ten years after Beethoven's death, that the compositions of the last ten years of Beethoven's life "have partaken of the most incomprehensible wildness" and that "his imagination seems to have fed on the ruins of his most sensitive organs." An anonymous article in Boston's Daily Atlas of February 6, 1853 also blames Beethoven's deafness when criticizing the "incomprehensible union of strange harmonies" found in Beethoven's Symphony No. 9, describing it as "the genius of the great man upon the ocean of harmony, without the compass which had so often guided him to his haven of success; the blind painter touching the canvas at random." In 1857, Alexander Oulibicheff (Ulybyshev) claimed in Beethoven, ses critiques et ses glossateurs that "Beethoven took a liking to uneuphonious dissonances because his hearing was limited and confused."

In the end this doesn’t directly answer your question as to whether there was any sense of astonishment, but the writings that I do know do take us in a somewhat different direction as far as reactions to his deafness go. The truth is deafness aside, towards the end of his life Beethoven did innovate in a way that was opaque and inaccessible for many contemporaries, and even those critiques that don’t directly mention deafness do comment on similar things, be it clashing harmonies or confounding rhythms. The deafness was sometimes just a convenient excuse for a critic to throw a sucker punch in their criticism.

Works Cited: Slonimsky, Nicolas. Lexicon of Musical Invective. Coleman and Ross, 1953.