I've just realized that
/isort condition armourychest category asc
/isort execute armourychest
gives the same result as
/isort execute armourychest
In other words, everything in the armoury chest belong to the same category, i.e. "gear".

Problem solved.

Now I would really love to have a role condition, that would sort gear according to what role it was intended for. E.g. tank, heal, dps.