Nope, it’s true. Although I think it’s quite a new thing.
As it was explained to me at the sim, BA have had some people struggle with a new type and they think it’s partly down to the infrequent nature of LH flying.
So in the sim they will assess how quickly you adapt to an unfamiliar type, the speed at which you learn and take in information, spare capacity, accuracy of flying etc...
At the end end of the sim the instructor will tick a box on your report whether they think you’re suitable for long haul, or whether they think you’ll benefit from some time with more regular sectors.
Also there was the caveat that if you get sent short haul, it doesn’t mean you failed the longhaul assessment, probably just that they need short haul people at the minute!
Its clearly not about flying skills as I got LH!
Its all explained when they go through the power point before the sim. Don’t lose sleep over it!