Agree with this comment.
It also (superficially) appears they haven’t studied deeper (more academic, research or UX driven) prior art. It’s easy to beat Preview or PDF Expert. Harder to surpass the more innovative examples.
In particular, there’s a lot of depth in Liquid Text:
But also a couple of the infinite canvas note anything apps, especially ones that also bring in audio recording or other ‘mixed media’ while covering swaths of the thinking outlined here.
Finally, there’s an irony that all the video embeds on this article are disabled on my iPad Pro. Do they even iPad?