I have seen that in timetables many times too.
You are absolutely right, the Tu-154 does not have the range for VVO-MOW non-stop.
If you notice the times for flights on the same route with different equipment (like the Domodedovo A/L IL-62 or Aeroflot A-310), they are about exactly 2 hours shorter... coincidence? just a slooow Tu-154? no way.
Its just that XF dont list the '1 stop' in the foreign systems and I have no idea why. I have asked my contacts over there about it long before now and they have no idea either.
I looked up the same flights with XF on www.polets.ru (sorry Russian only) and they list them as stopping in Abakan at the moment. In the past I have seen them stopping in other places like Irkutsk, Krasnoyarsk etc. too.
All of XF's flights to European Russia make a stop - remember their Tu-154 that crashed at Irkutsk about 2 years ago? That was coming from Ekaterinburg, with a tech-stop in IKT.
The only reliable place to get timetable info that Ive found so far is www.polets.ru.