I have the same hunch, to be honest. The core value proposition of Twitter, at least to me, is that it's short-form content - quick updates from the network that I care about. I don't go there to either write long-form content or consume long-form content directly (although I do get pointers to others' newsletter and blog pages).

My guess is that this is an attempt to replace long threads (1/n), but those have their own place and mechanics and I don't see how getting people to write Twitter Notes is in any way a 1:1 replacement or improvement for that.

Time will tell.