A big difference to me is that there is no gatekeeper for WASM like there was (Macromedia/Adobe) for Flash. Yes, it's true that you can't "view source" on WASM and get something meaningful like you could with JavaScript in the early days, but I'd argue that in the age of minification, most JS and the textual representation of WASM are comparably (il)legible.
I think the "gatekeeper" of Macromedia/Adobe is a bit of an overstatement; the proliferation of open SWF player projects shows it wasn't an obfuscated or challenging format. Additionally, they partnered with Mozilla to write the the JS engine for Firefox, then ES4 got scrapped. Adobe, having already built and shipped it for Flash, rebranded it as ActionScript 3. But it literally was a JS engine (what was supposed to become JS, anyways) and meant to be shipped in the most "open web" browser in wide use!
Also, before Atom and VS Code, Adobe worked on Brackets, which was a text editor built with web technologies (pre-Electron) with a lot of the same goals.
All this to say… it still wouldn't have been my preference (I love "View Source") but there's plenty of evidence they made attempts to engage the broader developer communities and do a fair bit of work in the open.
Good points, I wasn't aware of how supportive of third-party players they were. Was it possible to create .swf with open tooling as well? (I vaguely remember OpenLaszlo being a thing)
A little OT, but have you tried https://www.photopea.com for a "Photoshop in the browser" type experience? It's by no means a total replacement, but I've been impressed without how much is possible!