I'd say that angle actually shows it wasn't in at that point. There's no way that whole ball is over the line. That angle biases the attacking team, if you were to imagine the image rotated around the axis of the line where it meets the ball, you'd most likely see that a good portion of the ball is covering the line (even though there is a sliver of green between line and ball in the image). That said, that particular still frame could be the one taken after the keeper has pushed it back the way a little bit, we don't know.
What's funny is that the guys on Red TV were sure it was in because they take the reaction of the players into account. Because Nisbet was strongly claiming immediately suggested that it was a goal. On sportscene, later, the pundits were sure it wasn't a goal because they take the reaction of the players into account. Because the fullback was strongly claiming, it suggested that it wasn't in. Neither weighed up the reaction of the player who didn't agree with their opinion.
As you say, sympathise with the officials here. Luckily those types of call are rare. I think I'd probably have sided with Elgin in this case, but the linesman has to go with the view he has got. If VAR had been in play, we'd have been waiting 8 minutes, and they'd have just guessed in the end.