Bitsize of the MAC output of blake3

[elichai]

The crate provides https://docs.rs/blake3/latest/blake3/fn.keyed_hash.html as a MAC function, which returns 256 bits.
while:

• siphash is considered a somewhat secure MAC, even though outputting 64 bits.
• The HMAC RFC(2104) says:

5. Truncated output
A well-known practice with message authentication codes is to
truncate the output of the MAC and output only part of the bits
..... We recommend that
the output length t be not less than half the length of the hash
output (to match the birthday attack bound) and not less than 80 bits
(a suitable lower bound on the number of bits that need to be
predicted by an attacker).

So, is 256 bits necessary under the MAC security game? (where the attacker doesn't know the key, so he needs to attack not not blake3 but all family of blake3_key, so it sounds like some birthday attack between key space and output space) but I'm really not a crypt-analyst :)