a blocksize max for block creation of 300kb has been the default in knots for a long time
Then, I wonder, why early BIP-110 blocks used 4 MB limit?
began it two days early with hardly any notice
You expected a fair launch? Why should they do that? There is a lot of incentive to launch everything secretly, because then, early miners would have more BIP-110 coins to sell, and could have at least some chances, to cover their losses. They mined 8 blocks with BTC difficulty, and could earn 25 BTC, but got 25 BIP-110 coins instead. Also, it would really undermine their confidence, if they would have yet another halted chain, or something similar. Also, because the same algorithm is used by other altcoins, that risk is still there.
their fork doesn't implement any peer discovery of compatible peers
Which can be yet another reason, why their blocks are small. A similar thing happened, when
Bitcoin Purity launched, and had much less transactions, than BIP-110 chain with SHA-256.
By default, all nodes can only hear the current traffic. When many users are not running their clients 24/7, and are not re-broadcasting their own transactions, we have, what we have. People turn on their clients, send their transactions, see them displayed in block explorers, and turn off their nodes, because they think, that "someone" will keep relaying it. And in practice, when nobody cares about broadcasting it to new peers, then they ignore these transactions by default, and only some older nodes can mine them, or share them later.