In the thread I linked to somebody (tu_ki) said "Less than 177 pax block off rows 3 and 4". From other posts on pprune, tu_ki appears to be cabin crew.
According to seatguru.com, Ryanair's 738s accomodate 189. So that's only 23 empty seats before you have to do the row-blocking. That's going to feel like a full plane, particularly if 12 of those empty seats are in the blocked rows. at least 19 of the remaining 30 rows will be completely full.
It's also quite possible that once they'd shut the doors they realised that they had exceeded the magic number - I don't think the airline knows if someone (who has booked in online with no luggage) has turned up until they present themselves at the gate.
(Rows are numbered 1-33; there is no row 13 and row 1 only has 3 seats).