Okay and back on topic, it looks like a showstopper bug has shown up in their segwit2x code.
They've hit the >1MB hard fork on testnet which refuses to accept a block unless it is bigger than 1MB:
https://github.com/btc1/bitcoin/issues/65At this point there aren't enough transactions to create a second block with more than 1MB of transactions on testnet, leading to a fork with 6000 blocks that aren't using the 2x code and the two forks not speaking to each other.
Nevermind. I seem to have misunderstood what -ck was saying.
Showstopper? Doesn't look like it. Code working as intended. On reaching specified block height, block must be over 1MB to be valid. There are not enough transactions on the test net to hit 1MB before block mined. Why not? Someone threw massive hashpower at the testnet all of a sudden. 6000 blocks mined in 24 hours. Troll? Extreme tester? Doesn't matter. When mining slows sufficiently such that transactions can accumulate, chain will fork. Actually sounds like a success case to me.