There was no competition to get into block space, so even a transaction that pays 0 fees could easily get into the next block

I think at the start there was no fee.

Some miners can put their own TX in for free when they win a block. One of them famously donated 1 BTC to Luke Dashir from a 10000 BTC UTXO and paid no fee.

It looks like they were mining near-empty blocks and they excluded even the Coinbase transaction.

Maybe it was a miner doing some experimenting.