<"How do you have a median time to event x when you have no events?">
There are no "very late thrombosis" events with BMS, but there are still thrombosis events. I'm not sure what cutoff they are using here (perhaps one year) to say what is a "very late" thrombosis, but they are calculating the mean from the episodes of thrombosis (all earlier than this cutoff) that do exist.
""with no events in bare metal stent patients" and "The median time of late sirolimus stent thrombosis was 15.5 months, whereas with bare metal stents it was 4 months.""
In the meta-analysis, BMS had late stent thrombosis, but no "very late" (>1 year) stent thrombosis. That is the explanation.