Yes, the core of blake3 is a modified blake2b with fewer rounds, and yes the person who wrote the "Too much Crypto" paper is one of the blake authors, but there are other aspects of the construction that contribute as well.
Blake3 operates in a merkle tree mode using the modified blake2 as a node hashing function, which, in and of itself complicates attacks for reasons explained in the blake3 paper. Blake2 also didn't really have that many rounds to start with, Blake3 only reduces the round count from 10 to 7. A good _most_ of the speedup in Blake3 is due to the change in construction, the merkle tree mode allowing for unbounded parallelism, and not having the SIMD-friendly version be a variant. The parallel variants of Blake2 are already _almost_ as fast as Blake3 even without the round reduction (hell, BLAKE2sp can actually be faster than Blake3 in the right conditions).