Sort of. As you say, it's more accurate to say that they're forced to use Safari's rendering, but everything else is up to them, the same as how any other app would be developed. That's how they get their own features like bookmark syncing etc.

Being able to have multiple windows of the same app is a feature Apple introduced in 2019, and obviously Safari supported it immediately. Google Chrome added support for multiple windows after a few months. I switched to Microsoft Edge once they added support for it about a year, maybe 18 months later, and have just been waiting for Firefox to finally support it so I can switch to that.

Incidentally, 2019 is also the year Firefox finally added support on their desktop browser for a CSS property (column-span) that a site I used to frequent required to work. Though by that time I no longer used that particular site.