You are partly right. I say 'partly' because there is one use of "sit" where it helps form the passive mood:
(1) They were sitting at the back near the exit.
(2) They were sat in their seats by the usher.
(3) They were sat in good seats close to the stage.
In (1) "were sitting" is a present progressive construction denoting a (relatively) temporary activity in progress.
(2) is a passive construction with "were" + past participle "sat". It means that the usher helped them to their seats and to sit down.
By contrast, (3) is non-standard, mainly found in informal use only. Here "were sat" is a special use of "were" + past tense "sat", where it has an adjectival meaning rather like "they were relaxed/comfortable/happy in good seats ..."