I assume that no one named Louis has been canonized since C++ was invented though, and even if it was, using "the" might not be enough to disambiguate in some contexts.
I can say "No, it was the Saint" and "No, it was Saint Louis" but I don't think I'd utter "No, it was the Saint Louis" except as a speech error, maybe prompted by rushing like a Colemanball (commonly sports commentators mix metaphors or change their minds about intent midway through an utterance as of course they're live and events are unfolding as they speak, e.g. "That was a decisive mistake although nothing is decided yet").
I don't think you're wrong per se, but this seems more like a personal style of speaking rather than a matter of correctness; some people might prefer conciseness, whereas others might speak more formally and avoid any implicitness. I think it also can feel quite a bit different depending on how the words are emphasized; I italicized "the" in my previous comment since it was the word being questioned, but it doesn't feel super jarring to me to add "the" when "Saint" is emphasized in your quote above. I can totally imagine myself or someone else being confused for a bit and after in the relief of finally understanding dragging out the words a bit like "ohhhhh, it was the _Saint_ Louis, I thought you meant the _other_ one".