I find myself very much in two minds on this discussion!
Going back to the OP, assuming that the train was announced and shown of information screens as the GWR service to Cardiff, why would passengers going to Cardiff not board it because the wording on the train was (in this case slightly) different? But OTOH we should be aware that the railway can easily be confusing to non-staff or enthusiasts, as examples from the last two weeks show;
I have travelled twice on Avanti's 0736 Glasgow Central-Euston service, and on both occasions, when tickets were being checked at the platform entrance at Central, passengers in front of me were turned away because they were booked on Avanti's 0800 to Euston via Birmingham; In one case the passengers were travelling to Sandwell & Dudley, which the 0736 of course goes nowhere near. And on the second trip, now sat on the train at Central, a passenger boarded with a reservation for a seat in which somebody else, with a reservation for the same seat, was already sat! Luckily the Train Manager was passing and quickly found that the second passenger was travelling to Birmingham and was therefore, of course, booked on the 0800. Fortunately they were able to detrain (just) before the 0736 departed.
Therefore the railway, and we, should not assume that things which are obvious to us will necessarily be the same for others.