That is not the case - see the
Timetable History listing for one of the affected services (10:29 Shrewsbury to Aberystwyth).
It was published as part of the normal December timetable on 19 September last year, along with the rest of the Cambrian line service. Only on 28 February - 5 weeks beforehand - was the train cancelled.
That's considerably later than the normal engineering work alterations, even with the continuing shorter-than-usual timescales of roughly T-8 that NR and the TOCs are working to.
It's entirely plausible that people would have bought tickets - indeed perhaps even Advances - for the 10:29 etc. between 19 September and 27 February in the expectation that the train would run, as published. As a minimum, any such passengers should be able to claim Delay Repay based on any 15+ minute delay experienced.
Of course the industry sees it otherwise and a claim for Delay Repay would undoubtedly be rejected on the spurious grounds that the train was "never planned to run", as you have suggested.