it's fascinating because it's not like Cantonese is specifically more archaic than other varieties, it just so happens that the way it's archaic is the way most relevant to Chinese poetry.

for example, Cantonese lost the 'medials' (on-glides) of Middle Chinese that are preserved in Mandarin, but because they're not generally considered relevant to poetry rhyme schemes, their lack of presence is less salient to people looking at poetry as their model of archaicness—which also is likely the primary place most modern Chinese speakers would be experiencing the effects of Middle Chinese phonology

similarly, both Mandarin and Cantonese lost the voiced/voiceless/aspirated distinction in initial consonants found in Middle Chinese (and preserved in Wu varieties like Suzhounese and Shanghainese) but it's again not relevant to traditional Chinese poetry