I agree with you. The clincher is the line, "They've been blood enemies for seventy-five years." If they weren't something other than enemies immediately prior to that, then there would be no point in pinning it down to seventy-five years, give or take. By the way, that line occurs in "Reunion."
Moreover, as you said, that time is proximate to the events of "The Enterprise Incident." That's clear support for the position that they were allies at the time of TEI, for however brief a period.
Exactly. If Geordi had said "two centuries" or so instead of the very specific "seventy-five years," then it would be a non-issue. And also, "Redemption" was filmed six months before TUC, so at the time of the episode's writing, the only thing we knew about Romulans other than that they were using Klingon designs in TOS was that they had a representative visit Nimbus III starting in 2267.