It’s quorate and probably legitimate (if not entirely moral) as it sounds like the Strata Manager has delegated powers as secretary. If you also have an elected secretary you can ask them to over-ride the SM’s meeting plans.
Or you can ask just one of the other EC members who can make it to say they won’t attend if the Chair and Treasurer aren’t there – then it won’t be quorate. Basically, you should come up with dates when your chair and treasurer and most other members can attend and let the SM try to fit in with your plans, within reason, rather than the other way round.
Strata Managers will hate me for saying this but it’s a case of the tail wagging the dog, otherwise.