As so often in life: there is no line! If you are immersed in the game then it is, really that simple!
“Immersion” itself doesn’t have an absolute clear definition, people sometimes mean “feel like a part of it” and sometimes “getting lost in”. Both is fine :)
For games specifically it’s along the lines of “what does a majority feel”? To give a specific example: Legend of Zelda is often described as an immersive game - and I never saw why, just accepting that for many people it seems to be.
At least in my version, it’s still lake Ontario. And also gulf of Mexico, btw.
That doesn’t mean that Google isn’t a dipshit company - but wanted to let you know that in that regard they’re consequential fuckers (non US version for me).