I think TfW Rail are stuck between a rock and a hard place. They are required by the Welsh Govt to ensure that people can travel in a covid-safe manner, but have little to no powers to enforce anything. Their only option seems to be to make their messaging as off-putting as possible. If the number of passengers returned to even a fraction of normal numbers, there is no way they could ensure the social distancing still mandated by the Welsh Govt.
I was expecting the Welsh Govt to drop the 2m rule, or at least reduce it to 1m, when the wearing of face masks became compulsory. However, the govt are hardly likely to encourage even more people to travel by train when things like the Barry Island incidents are happening right on their doorstep.
I'm sure it's not easy for them. However I'm not sure the situation is as bleak as you suggest - there were plenty of services that with the same loadings as in the past would be fine now if they could adopt the policy which bus companies do (only one person per double seat unless from same household) - and of course a lot of leisure travel will consist of more than one person per household so you would end up quite a bit above 50% seated capacity available. And while some services did run out of seats pre-Covid, on the ones I've used it was generally commuting time, Barry Island on a nice day and when events were on.
And that's without the fact that
some people on those trains in the past no doubt could have driven, which they are of course now asked to do. And while there are fewer services than before, that should give more scope to run the trains that do run as longer ones in many cases.
In any case they aren't justifying it directly on the fact that they can't accommodate all the would-be passengers - they're just saying that the government says that public transport in Wales is only for key workers and essential journeys.
(Though in which case, who sanctioned the TFW FAQ which says that leisure travel is now OK?)
One solution to Barry Island might be to recognize that people are going to try to travel no matter what they say, and perhaps even that it is reasonable for families without cars to be permitted the odd day out in the summer holidays even in these strange times, and lay on dedicated transport for it.
I have no idea whether hire coaches could be arranged for such a purpose.
Finally, I still remain unconvinced that the best approach to the situation is to be so aggressive in their messaging towards prospective passengers, and to fail to acknowledge that banning people without cars from leisure travel is actually a bit of an imposition - even if it is necesary.
The time may come when they actually want to attract passengers again and I can't see this helping.