Well yes, but perhaps they are not deliberately splitting parties up, but blocking certain seats from assignment to non-payers, until departure time. These seats could very well include all window and aisle seats, which are more in demand than middle seats.
At the time of check-in, often weeks in advance with the lcc model, two seats together may not be available. Closer to departure, as the flight is managed, adjacent seats could become available, but by then it isn’t easy to reseat allready checked in passengers.
If I am booking close to departure, and am prepared to pay for seat selection, I certainly don’t want all the aisle/window seats to be allocated to people that haven’t paid.