I've read Sect 26, it talks generally , -what is needed is a pin table, like microchip do. Im not going on at yourself, its Atmel, all they have to do is call a spade a spade.
I beleive, from reading a lot of vague , general data in the atmel datasheet that PA20 and PA21 are push pull drive capable?
I have no idea why a consultant engineer is saying he thinks these pins are open drain. I can't see anywhere in the atmel datasheet where it says they are open drain only....it appears that all I/O pins can be configured open drain, but dont have to be that.