The plausibility of this scenario is that The Mandalorian was just a child when he got adopted and the Jedi were massacred shortly after. And of course the empire did whatever it could to remove all traces of the Jedi.
And although Luke is a famous figure int he rebellion this is a fast galaxy and it would take decades for him to become a household name compared to high ranking generals or political leaders like Ackbar or Organa.
Why the Mandaolorians specifically don't know what a Jedi is comes from the point that they rebelled imperial rule and got massacred. This would leave major gaps in their history.
But you are completely right int he fact that doesn't make any sense at all. Seeing it's suggested (Due to her position in the order) she is probably at least a decade older then the main character and therefor should have even witnessed the clone wars as a teenager/young adult. and same goes for some of the other members. So unless there was a form off mass amnesia...
Sadly it seems Disney is to blame...sometimes i got the feeling that the ones producing the new stories never looked at the expanded lore of the series before. So retcons(which have been mostly positive in the series at least) and inaccuracies are (going to be) common i'm afraid.