I'm not a resident so I don't actually know for sure. However, I rather doubt passing MOT depends on the driver's seat not having any cracks.
Its a question of how soon in a cars life you willing to tolerate aesthetic imperfection on its the upholstery. Small splits like this, in this location, are never likely to fail an MOT or other technical inspection. But it may affect its resale value.
But I have heard of a very high mileage foreign registered van being seized by the police whilst transiting through Austria because they considered its upholstery was in such poor condition it didnt hold the driver secure and was dangerous. (maybe the seat was virtually collapsed with loose foam rubber stuffed into gaping holes etc) .
But it is possible to fail the UK MOT (and probably other countries) if the seats are insecure, or have broken metal frames etc. So maybe the seat condition was actually worse than the driver claimed on a forum when he sought sympathy and the condemnation of over zealous police. (although Austria do have a reputation for strict traffic enforcements and large fines. )
Speaking of disguising cracks with marker pen etc has anyone found a flexible sealant that might be a good colour match for filling cracks. . Years ago I had some black acrylic filler (not silicone) in a squeezy tube that might have been suitable.