There is a way to test the validity of such an assertion by the clerk - but it is a bit radical!
Don't attend meetings for 6 months, send apologies via a route other than the clerk and then see if the clerk / council can legally / realistically seek to have you removed.
Joking aside, you wouldn't actually have to do that, but you could present it as a scenario to the clerk and say - go on then, provide the references that show this would even be possible / viable. If they can't, then by exclusion, their statement that apologies are only valid if received by the clerk is shown to be nonsensical and the point is proven.