Considering the site I'm posting this on, I'd say it's safe to say a high majority of us here have watched our fair share of game playthroughs. And while sometimes it's of a game many of us have played or beaten already, there are lots of instances where you are watching a playthrough of something you've never seen, played, or beaten before.
Which leads to the title question, when you have watched an entire playthrough of a game you haven't played/beaten, do you consider that similar to having beaten the game? The more mechanical the game is in nature, the less relevant this probably becomes, especially when it all comes down to feel and challenge. But so many games now, even less narrative focused ones have progressive mechanics that absolutely do translate when your watching someone else play them. You can see the difficulty curve, and often other mechanical changes get noted. I think Vinny's recent run of Bloodstained is a good example of this.
I often fall in the middle on this issue since it really depends on the game. I've never played a single Metal Gear Solid game, and I plan to keep that record in track. Yet if someone asks me, I mention having seen them all/know what they are about because of watching Metal Gear Scanlon. To me watching that series satisfies the same feeling as having beaten those games would. Obviously to be an "expert" on any game I'd imagine most would agree you'd have to play it to some degree. But what about pure narrative games like Until Dawn or something of that ilk?