I might have missed something, but is this car a Leon
Cupra that has been bought used from a VW
dealership, has run out of SEAT new car warranty and is now covered by a VW Used car warranty?
I'm surprised that the VW workshop diagnostic kit is any different to the SEAT workshop diagnostic kit - although the online connection will be to a different VW Group technical site, not too encouraging if there are too many differences to make VW sorting out a SEAT impossible.
But, if that car is still covered by the SEAT new car warranty, then it makes sense that any expensive workshop tasks are carried out in a SEAT workshop - maybe same if it is covered only by a VW used car warranty if the local SEAT workshop labour rate is
lower than that VW workshop.