Why does the computer want to undermine this pawn chain at the tip instead of at its root? I had the following position with black in a game I recently played:

[FEN "r1bqkbnr/pp1n1ppp/2p1p3/2Pp4/3P4/4PN2/PP3PPP/RNBQKB1R b KQkq - 9 5"]


and proceeded with 5... e5

However, after the game was over, while reviewing it with a chess engine, Stockfish suggested 5... b6 instead and gives -0.16 (equal)

I find this very confusing. I was taught in Nimzowitsch's My System (among in other sources) that one must undermine a pawn chain at it's root, hence e5 seems appropriate.

So my question is: without looking deep into variations, why is in this case b6 a good move (in principle)? Wouldn't white follow up with b4 and have a nice position?
You would like an answer that does not "go into deep variations", but if all such questions could be answered just on general principles, chess would not be as interesting as it is. In this particular case 5.c5 is definitely premature and BOTH methods of undermining the center work well, transferring the initiative (but no more) to Black. The tactics are important. AS Glorfindel observes, after 6..a5 the chain cannot be preserved. If it could, the assessment would change. Usually, though, you either need to strike soon, or you must be able to see that a delayed strike cannot be prevented.

But computer assessments at early stages of the game do not mean much. Stockfish is just saying that both approaches are playable, and you cannot trust it more than that.