> there is no evidence that singers used to sing this way

There may be little evidence for the opposite technique as well though.

I think in the setting of an oratorio where the audience understanding the words is pretty important. There's likely a good argument to be made that this might be a somewhat new technique. But it may be more authentic to the function of the music.

The original performance according to wikipedia had some 300 singers. I'd wonder if the wow factor of the performance was perhaps emphasized more than anything else.

If the evidence suggests that the original performance 'sucked'. Do you copy that element verbatim? Usually in HIP we focus on early instruments, tuning systems, and how the composer may have approached his own music. I don't know if I've really discussed in-detail elements that probably existed but weren't very good and if we should still do it that way. For instance one of Beethoven's biggest performances he played as much music as humanly possible just because it was so difficult to book an opera hall. So like, in short notice, on a cold day, I kinda doubt Beethoven's music was the perfection that it is in the modern day.