I wouldn't read too much into this. Apparently it is only a standby aircraft. That being said, I do think the Southwest region is edging ever closer to a Jet2 base. EZY have taken most, if not all of TCX's slots at BRS which means that slot quota is still pretty used up. That could be a deterrent for Jet2 going to BRS. They will know their customer base, and if they are attracting people to BHX from South Wales and the Southwest, there's no reason why they won't attract people from the Southwest to CWL. There's now less competition at BRS, so although in some degree this will effect CWL as well, prices at BRS will likely go up. As long as prices increase more at BRS than at CWL, it may discourage people from South Wales in booking BRS flights over CWL flights.
Whilst it's a huge shame to not see TCX anymore, I think TUI, VY and FR at CWL will do quite well out of this, which may stem growth.