Well, I respectfully agree to disagree depending on what the definition of "functional" is (since different people have different opinions on the definition of the term).
For me, I'm after overall strength. With around 18 years of training under my belt, I can safely say that nothing has ever given me the overall strength or carryover to other lifts/other activities. In fact, I was actually pretty weak overall when I was a leg press fanatic years ago despite being able to knock out 1000+ lbs. for sets of 8-10 any day of the week (not to mention, my first lower back issues were experienced when leg pressing, it's really unnatural for me as far as the feel since most machines are less than comfortable when I use them). There are quite a few contradictory experts who have noted that there is often great lower back strain for many people withe leg pressing - once your hips come up even a fraction of an inch, you're opening yourself to plenty of injury potential regardless, it's not truly any safer than a barbell squat when squats are done properly. This brings it back more to an exercise being safe/unsafe due primarily to HOW it is performed moreso than WHAT is being performed, as anything can cause injury if not done properly (I even strained my lower back about 15 years ago doing a set of EZ bar curls, proving that one second of carelessness in any lift can put you out for a bit).
Of course, it always seems the majority of people who squat really have little idea of proper form, so they often end up injured due to poor technique and end up blaming the squat when it is much more fault of improper form (usually due to people's desire to stroke their ego and pile on more weight than they're capable of).
It goes without saying, you CAN build good size without squats - plenty of people have done it, but suffice to say, the strongest (and almost as often, the fastest) people tend to squat as they find it more appropriate to reaching their overall goals. But sure, if someone just wants to build bigger quads and use machines, you can still get some big legs without ever squatting....but, squatting might just do more than leg pressing ever could, you don't know until you spend enough time under the bar!
I'll cap it off with this - I've read training logs from many people who have squatted hard and heavy for decades on end, many of who have never suffered the problems noted in the video. Heck, one of the guys at our gym has squatted 500-650 lbs. (belt and wraps only, no squat suits) for over 2 decades, he's in his mid-50s now and is in better shape and stronger than the bulk of guys half his age and has told me that not once has he ever been injured squatting, but ask him about leg pressing and he'll give you an earful about what it did to his knees early on in his lifting career. I've read quite a few stories from long-time strength competitors and various athletes who almost ended their careers with machine-based injuries despite the claims some make that machines are inherently safer. Food for thought!
"A 'hardgainer' is merely someone who hasn't bothered to try enough different training methods to learn what is actually right for their own damned body." - anonymous