If I had to guess, it's coming from people who have gone tribal about knots vs core and realized Calle's on the Core side of things.
Personally I'm getting more intrigued by how Fedi is addressing some of the trust trade offs, but for serving as my "mailbox" of sorts for zaps while my lightning node is offline, I can't complain too much about Cashu. It's only marginally better than normal custodial lightning, but at least I can see who zapped me now.