I think it's an "alter," because before "Yesterday's Enterprise," the last recorded contact with the Romulan Empire was the Tomed Incident, 50 years prior to the episode "The Neutral Zone." After "Yesterday's Enterprise," history is aware of the Enterprise-C and her sacrifice at Narendra III, putting the last contact with the Romulans much closer to the "current" time frame.
Except that "Angel One" featured a builiding confrontation with the Romulans as a plot point months before "The Neutral Zone" came along. So either you accept that there are simply continuity errors in this work of fiction, or you don't interpret the "no contact" line from TNZ too rigidly.
In fact, what Riker actually said in TNZ was that there had been "no direct contact" since Tomed. So there could have been indirect contacts -- such as a Starfleet vessel defending a Klingon outpost from a Romulan attack, or such as whatever happened in "Angel One." I tend to interpret the line to mean that there was no formal diplomatic contact between the governments.