Is it secure to repeatedly rehash a digest to extend the digest length?
Section 2.6 of their submission talks about this.
There they state that for most cases they simply use SHA-256, -384 or -512.
In the (uncommon?) case that a hash function with a larger output is needed, they use the OFB-like1 construction.
The rationale being that a collision on the OFB construction would immediately imply a collision on the underlying hash function - as the direct output is part of the expanded output.
This also explains why they don't want to involve another (more efficient) function: To reduce security guarantees and prefer simplicity over optimization for rarely needed scenarios.

1: OFB-Mode is a way to construct a PRG and an CPA-secure encryption scheme out of a fixed-length PRF. It works essentially as $K_i=F_k(K_{i-1})$ outputting $K_1\|K_2\|\ldots\|K_\ell$ for a PRF $F$ and a secret random key $k$.