It's not a mistake because someone doesn't subscribe to the same definition as you. There are 2 widely competing definitions that are both perfectly valid, if you want to be more specific you can say "not OSI approved" to more accurately reflect what you're talking about, if you don't want to do that then you can understand how others feel.
I used to make a bigger deal about this, but now I think that whether something calls itself Open Source is less relevant than if bait and switches are being done.
Teasing a release on X is less bothersome than what Matrix is doing by relicensing from Apache to AGPL and making what was billed as a vendor neutral communication platform not so vendor neutral. The people working at Element certainly don't want to use Matrix/Element under the AGPL, so why should they expect earlier users and members of their community like me to want to use it under the AGPL?
There was a time when saying Open Source meant something by itself. Now you have to include details like the license, what exactly is under the license, and the leadership.