I'd call it a permanent chain-split caused by a soft fork with minority hashrate support. The Bitcoin rules would be the hard fork to RDTS, but RDTS changed the rules in a compatible manner and Bitcoin didn't, so calling it a hard fork would be weird.
Hard/soft fork refers to the type of rule change. Chain split describes the outcome.
I'd call it a permanent chain-split caused by a soft fork with minority hashrate support. The Bitcoin rules would be the hard fork to RDTS, but RDTS changed the rules in a compatible manner and Bitcoin didn't, so calling it a hard fork would be weird.
Hard/soft fork refers to the type of rule change. Chain split describes the outcome.
I wrote as much a long while about it, so that distinction seems to be quite old: https://bitcoin.stackexchange.com/a/30821