Why in meetings, and when classes finish the teacher would say “dismissed!”, but not “dismiss”?
Dismiss in imperative is not a command to go away but telling people to dismiss someone.
I want you to fire him. Dismiss! Now!
Dismissed is not imperative past tense, just past tense.
"Dismissed" is short for "You are dismissed", e.g. you may leave. Not "Get out of here". It is a request - possibly sounding like a command, but that does not make the word imperative.
The sentence could be said to be in the imperative mood, but I am not certain of this in your example.
To the best of my knowledge - If I 'dismiss' (present tense) somebody's opinions I choose (deliberately) to ignore what they are saying. If I say you are 'dismissed' (an action taken and decided already) it means you are being suspended temporarily or effectively sacked or fired - made redundant. 'Dismissed' as per dictionary and use of English may be referred either to one or more than one person. If I 'dismissed' his argument as fallacious - it means I would not consider his/her opinions.