Even if all miners immediately move to B2X it doesn't make Bitcoin less secure for at least months.
The hashpower driven security of bitcoin is determined by the current difficulty target. This target only changes every 2016 blocks or every 2 weeks under normal conditions. With a significant chunk of hashrate suddenly missing, it could take several months to reach the next difficulty adjustment. Even then, the target doesn't change more than by a factor of 4 in either direction. So even when we readjust, it will set "Bitcoin's security" only back to late 2016 levels.
Miners can't hold out that long in case bitcoin is valued higher than bizcoin. Which is very likely to happen because a) all the development brain power stays with the real bitcoin and b) bitcoin will be short in supply on exchanges, thanks to the inability to deposit due to a very slow blockrate.
They literally can't win this.
Worst they can do is cause an inconvenience for bitcoin users (only for spenders, hodlers wont really care).