Today's short forms have absolutely nothing to do with the 153s heading to Wales. 4 x 143s and 2 x 150s were out of traffic for various reasons, along with a couple problems with the 255/257 west HSTs. With the injection of some of these 2+4 HSTs onto Cardiff-Taunton diagrams, and subsequent displacement of Turbos and therefore 15x further west, there are is actually more slack in the west fleet than for some time.
Let's also not forget that TfW's needs for rolling stock in the Cardiff area are actually quite acute - if GWR can manage for a few more weeks without them, before further Turbos head west (displaced from Bedwyn by IETs), then it's probably sensible overall.