Haven't heard of that one before. There are 6 floor mounting points for each seat: four bolts in the corners of the seat rails into welded nuts in raised channels above the chassis and two thick studs into keyed slots. I would expect any three might be able to hold the seat except in a severe accident. It really does make you wonder if some of the mounting bolts were missing. About the only way I can imagine those mounting areas getting that badly rusted is extended exposure to interior water, especially water with road salt, and the carpets under the seat staying wet for extended periods. Floor mats can keep the footwell area carpeting from drying out, possibly giving a chance for water to wick into the carpeting under the seats.

Any chance this was a flood car? People will ship flood cars out of recent flood areas so as not to raise buyer suspicions. A Carfax report might have given you a better idea where it's been driven.

Water can get into the cabin through failed door seals, window seals (especially the windshield corners after glass replacement!!!), doors out of alignment, blocked sunroof drains (!!!) and even leaking heater cores and leaks through the firewall. Rust and floorboard perforations in the front footwells are not that uncommon in 140s and 240s and could allow road spray in to soak the carpets. If left to get bad enough, welded floor panel sections may be needed. Lift the front carpets and check under the padding for rust and perforations. The rear footwells are less prone to this, but should also be checked.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Only way I can see that happening is if the mounting bolts and nuts of the seat frame or rails all came out or some were not even there.