“Mountain biking (not including downhill racing and extreme ground conditions
^^^ wording there is identical to Travelinsurance.co.uk (which I’ve got) which is owned by InsureandGo (so probably same wording as theirs too).
On mine, while you can pay extra to include “extreme ground conditions” it also says you won’t get personal accident and personal liability cover, unlike with the standard MTB cover (which comes down to what the extreme ground conditions actually means. I’ve read some were told this is anything but tarmac.
Also I think I read something about not being covered for a dedicated sports holiday, except if it’s winter sports and has that optional cover. i.e. only covers for taking the odd day out to do some activity.