Link is below to the original blog post, github link is in that post.
So, the PET and others use a type of FSK. I tried that originally as well. It's not only slower but not as reliable I think.
I think perhaps it was easier to do fsk back then? Idk.
But in the discussion you'll see the benefits of manchester are I think we'll suited to this medium.
http://zeninstruments.blogspot.com/2021/10/manchester-decoder-and-cassette.html?m=1