Rote memorization without understanding the "why" is useless.
Why MEMORIZING Openings is BAD!

Yes, what's surprising about that?
The point is, if White, which was me, only memorized openings, the correlation between ideas for White in the French and this game, which was a Torre Attack, would never have been understood. You'd think ...c4 was fine when it really wasn't. You'd think all was fine and not realize your significant positional advantages. The point is realizing similarities in pawn structure, yet also finding the minute differences that make a significant difference in the position, like White's pawn on a2 instead of a3 making castling queenside a bad idea for Black, so he had to go kingside where White has a local piece superiority, and that wins him the game. Almost every idea White had after the e4 push were akin to that of the White side of the French Defense.
So why are you shocked Black lost? He played a terrible move in the opening, and his subsequent play was just as bad, ramming pawns when he needs to be developing his pieces, and so pieces like the a8-rook, b7-bishop, and c7-Queen, contribute ZERO to the defense of Black's king and Black drops 2 pawns followed by the game.

Memorizing is inconsistent and will sometimes fail you, understanding will let you go the extra mile.

Obviously it's important to understand things you memorize. Although it's also very hard to memorize things without understanding them. To manage to learn the french without acquiring some understanding of the ideas would be very hard, you'd probably have to deliberately try to do that. And besides... when you memorize even smaller combinations you acquire them as patterns and they recur.
Your example doesn't really illustrate your point, since as a french player you had the french very well memorized, and as a result were able to recognize the pawn structure and how to proceed. If you had never memorized the french you would not have recognized it, so...

After 2.d5 if 3.Bxf6 it's basically the Trompowsky Attack. Nd2 is also a decent move, other than Nf3.

Just to add an important note:
...c4! would lead to a nice advantage for Black, if black's move had been 7 ...Qb6 (let's say, instead of 7...h6) here, and White responded to it with 8 Qb3 question mark.
Since after Qxb6 axb6, the b-pawn push to b4 would be unstoppable due to the tempo on the d3 bishop, and then to the a-file pin, which leaves White no chance to move the rook off the a-line and play a3, plus then black has a second b-pawn in reserve!
So I am playing in a 5 round weekend event, and round 1 was tonight. I won fairly easily. Why? Because of the UNDERSTANDING, NOT MEMORIZING of a completely different opening. Same ideas crop up in other openings.
This game starts out a Trompowsky, directly Transposes to a Torre Attack, and yet, plays like a French!
The opening may have been a Torre, but the game played itself like a French, illustrating the importance of understanding openings rather than memorizing them. Understanding the ideas of the French lead to a win in the Torre Attack!