Regtest has neither DNS nor fixed seeds. The net.cpp 1229-1237 block in ThreadOpenConnections() acts oblivious to this, and attempts to add fixed nodes, assuming DNS seeds are down. While this may not necessarily be entirely inappropriate (in terms of 'path invariants'), this also may not remain harmless indefinitely, if we are assuming the possibility of an 'infrastructure attack' as the comment does (legitimate in the context of mainnet, or even testnet). At the very least, the comment itself needs to be updated to call out the regtest use case.