> To be fair it’s fairly easy to check that it didn’t send any data over the wire

... after the fact ...

... for a user of the web app who watches the network panel from their browser's developer tools

You just send false info first.

Well, maybe the site sends all the passwords when you close the tab. The point is not to get used to entering passwords to third party websites.

"just"? The above comment is overconfident.

Yeah, 99% of the society also knows how to do that.