But the developers ignored this when they designed this quest. For the purposes of this quest, Hammerfell is still as it was circa 4E 175 not 4E 201. This is why Kematu refers to the Redguard resistance being alive and well in Hammerfell. For the purposes of this quest that statement makes no sense at all if it were otherwise because the resistance hasn't existed for over 20 years.
If you don't accept this then Kematu is clearly lying and you've roleplayed your character as in ignorant, uneducated, easily guiled fool. Saadia is in her twenties to early thirties in age. The fall of Taneth to the Aldmeri Dominion for which Kematu accuses Saadia of colluding with the Thalmor occured around 25 years ago. The notion that Saadia would have played any kind of willing role in assisting the Thalmor as an infant or a toddler is ludicrous and yet your character has just bought into the absurd notion which was relayed to you by a paid-for-hire mercenary who employs criminals (bandits) that are responsible for robbing and murdering innocents on the roadways of Skyrim and whose fellow mercenaries harass every Redguard woman in Skyrim to the point of illegally kidnapping them and are condescending and completely unapologetic to the same women after they realize they're totally wrong.
You're much better off accepting the fact that the developers f***fed up regarding the lore, suspending adherence to it for this quest, and accepting that for the purposes of this quest you've temporarily stepped into an alternate reality Tamriel where Hammerfell is as it was circa 4E 175 even if it means you have no compelling reason to believe either Saadia or Kematu's version of events.