The bit ‘the supervising driver will usually be covered on the learner driver’s own insurance’ can be read a couple of ways…
Perhaps it means that when you’re supervising the learner, in the learner’s car that is properly insured for the learner to drive, that any liability arising from a mistake by you the supervisor is (usually) covered?
For example, the supervisor tells the learner it’s ok to pull out of a junction, when in fact it wasn’t and a collision occurs. The supervisor, being deemed in control of the vehicle, is at fault, but their third party liabilities are covered by the learners policy.
Alternatively, ‘ the supervising driver will usually be covered on the learner driver’s own insurance ‘ could be read as the supervisor is usually covered to drive the learners car, but it’s not explicitly saying that.
I think it’s the former, although also think its a little ambiguous.