Take Google Sheets as an example. I don't know what the total download size is for the JS (certainly not 15mb), but it's an example of a JS heavy website where you wouldn't want to serve -> submit form -> serve cycle for every user input.
This entire discussion started from an opinionated blog post that directly discusses AMP and it's performative nature on the web.
So yes.
It is.
You can't just invent a hypothetical that makes it easier to justify exceptions to the very topic the original blog post we're talking about here, of course exceptional things like resource-heavy, full-featured web apps are going to be outliers to lightweight payloads for web apps. For some reason I highly doubt those were the types of use-cases Google had in mind when AMP was released, so why are we even considering exigent outliers on this one?
Since when did forms become useless and what web-based video editor are you (or the original reply-er) pointing to that uses AMP?