Correct. If you request, the maximum reported here is 30 days and at present people do seem to be getting that amount of time.
Assuming that you aren't wanting to spend more than 8-10 days in Tibet, then you should be able to take a train to Beijing, spend a few days there and then depart for Mongolia and Russia before your 28-30 days expire.
Train from Lhasa to Beijing takes just under 48 hours.
Train from Beijing to Erlian (border with Mongolia) only departs twice a week but there are daily overnight buses. Trip takes about 12-13 hours.
If you take the train, you'll have to spend the night in Erlian because the border is only open during the day. This post from summer 2007 by manulike with update from Nov 2008 has useful information: Erlian Fact Sheet.
Or if you are heading directly for Russia, you can take the train from Beijing to Manzhouli. I'm not sure how often it operates but the K274/K275/K277 takes about 31 hours and other options are a little longer. Again border is only open during the day.
Of course, I am assuming that you are heading directly home...
Ruth