pathfinder 1e – If a swashbuckler rolls a nat. 20 on Opportune Parry and Riposte, do they automatically parry attacks even if they do not beat the attack roll?

No

Opportune Parry and Riposte is not an attempt to ‘hit’ anything. They include that it is an

attack roll as if she were making an attack of opportunity

but then goes on to state

If her result is greater than the attacking creature’s result, the creature’s attack automatically misses.

It is an attack like an attack of opportunity for the purpose of abilities (feats, spells, etc) that affect those things but your only goal is to roll greater than the target (the rare exception to ‘meets beats’), not hit anything. Your GM may wish to allow an automatic success, but there is nothing in the rules that specifies that it works.