That would apply to a multi-entry visa that's already been presented.
AFAIK, the MFA doesn't notify the RTP when they issue a visa and there's no reference in the RTP database if it hasn't been presented yet. The IO scans the passport first, and all previous activity linked to the passport holder's biometrics will be displayed. Then they start flicking pages. My experience is that some IO's will ask at this point if I have a visa, probably to speed things up (you are doing their job of scrutiny for them). That's maybe because my history shows a mixture of entries as visa-exempt, on a visa or on marriage/retirement extensions. There habe been several posts, including on this thread, where people have either managed to get a visa-exempt entry while holding an unused visa in the same passport due to the IO not checking properly, while a few others have tried (with varying degrees of success) to get the IO to ignore the visa and stamp them in on the visa-exempt allowance.
My understanding, based on my experience and what I was told by an irritable and irritated Immigration 'boss' at BKK, is that if one has a visa in any travel document, that is the only travel document and only entry entitlement that can be presented for entry to Thailand.
Good luck to the OP on getting the less observant IO if he tries to get a visa-exempt stamp. Make sure it's a bloke, as the ladies tend to have eagle eyes.