A third conditional expressing uncertainty?

Appian wrote that Hadrian, like Titus, destroyed Jerusalem, implying that there was something to demolish, and the rebels, sweeping all before them, would surely have besieged the Tenth Legion in the Citadel and worshipped on the Temple Mount if they had had the chance, but we do not know if they did.

Excerpt From
Jerusalem
Simon Sebag Montefiore

Hi. I think the underlined sentence implies “they(the rebels) didn’t have the chance so they didn’t besieged ... and worship ...”. This implication seems to contradict the next clause “but we do not know if they did”, which suggests uncertainty. Why can a third conditional express uncertainty? Could you please explain that?

Thank you.