I've always interpreted Picard and Riker's statements as broad generalizations about the Romulan's absence from galactic politics following the Tomed Incident. That they weren't dealt with by the UFP for fifty years doesn't mean nothing happened during the time in question. The USS Intrepid responded to Khitomer's distress call and arrived following the attack, but we don't know if they actually encountered Romulan ships.
Based on "Sins of the Father," it sounds like the Khitomer outpost was wiped out fairly quickly (the shields were down). The Intrepid was at the edge of sensor range when the attack occurred as indicated by Data's review of the logs from the day of the attack. There's no mention of their having sighted actual Romulan ships. They probably ID'ed the Romulans as culprits based on residual weapons signatures and any Klingon sensor data that was salvaged from the Khitomer wreckage.
The Enterprise-C encountered at least four Romulan ships while trying to defend Narendra III from attack. It's not clear what the Romulans were doing in that system, nor do we know if their failure to destroy the outpost was in any way related to the eventual attack on Khitomer, as this happened two years later. Aside from these incidents, the Romulans seemed to have dealings only with the Klingons while they were attending to internal matters. Given the proximity of Klingon space to Romulan territory, it's not a surprise that the two governments would continue sparring even as the Romulans sought to isolate themselves.
For all we know, the isolation was brought on by repeated incidents with the Klingons. That the Federation was involved with the Tomed Incident may have been the last straw, but that doesn't mean there weren't other things that led up to it.
