Because length extension attacks only matter if you're doing something you should never be doing: hand-rolling your own keyed hash constructions. Everybody uses HMAC (you even see it in SHA3 systems), and so length extension doesn't matter. Meanwhile, SHA2 has hardware support.

I like BLAKE3 just fine. Nobody's really going to ding you for using it (or SHA3 for that matter). But it would not be sound practice to tear SHA2 out of a system that was already using it.

Two other notes:

(1) The article explicitly recommends HMAC-SHA2, not SHA2 by itself.

(2) Where it does recommend SHA2 directly (for hashing, not authentication), it recommends 512/256, which is truncated and can't be length-extended.