Wouldn't read much into the "timed for 75 mph" bit, probably the same arrangements as the two Padd Cardiff LHCS trains that ran yesterday formed of hired in sets of Mk 2f stock from Riviera Trains with top and tailed 67s.
It seems GWR are able to procure stock to supplement their services from other sources, where XC are unable to use their own.
Or maybe it just wasn't practical, given that the XC HSTs not used on a Saturday are miles away from Birmingham and Cardiff at Plymouth, Leeds and Edinburgh and the ones in the north may be under maintenance.
They all need to be back in those places for Sunday's service, when four are in use (so not available to go to Cardiff). How many miles of empty running/crew hours do you think that would add up to?
On Sunday evening, XC ran the following services out of Cardiff, which hardly suggests they weren't trying:
17.26 to Manchester, 10 cars, presumably 2x221
17.45 to Nottingham, 5 cars
18.14 Cardiff-Birmingham, 5 cars
18.45 to Nottingham, 3 cars
19.45 to Leicester, 6 cars
20.45 to Birmingham, 6 cars
Plus there were five ATW services to Gloucester or Cheltenham during the evening.