>Dr Birley said many examples of stone and marble toilet benches existed from across the Roman Empire, but this is believed to be the only surviving wooden seat.

So the article you link says, in colloquial English terms, that stone and marble toilets from the period are pretty common but wooden toilet seats are basically all disintegrated. Only one (1) wooden seat is known to remain.

That some individual artifacts in lucky conditions have survived does not invalidate the statement that most don't

having been there does. there is no shortage of old wood and pottery

believe fewer unevidenced claims

Unevidenced...?