I am not an insurance expert but I think the best insurance cover is for a 'waiver of subrogation' to be in force for the pilot. This is because most pilots do not own the aircraft they fly and therefore the insurance is principally for the owner of that aircraft. If the hapless pilot has had a mishap and found to have erred in some aspect of the operation eg run out of fuel, medical lapsed yesterday, that kind of thing, then he may very well find that the insurance company will cover the financial damage for the owner of a bent aeroplane and/or occupants and then pursue the pilot for recovery of a similar amount. This is called 'subrogation'. A waiver of subrogation (you need to see the policy document) would generally avoid this unpleasantness. In this regard, I doubt whether or not being an instructor would make a scrap of difference.
I no longer instruct, but when I did instruct I did not have any liability insurance - and by that I mean to fend off financially an unhappy Joe Bloggs alleging, for example, that I did not train him correctly in forced landings, that he had now crashed and was demanding compensation. Basically I kept my fingers crossed and I suppose I got away with it.
However, in this increasingly litigious world, I would not put it past an individual many years into the future to come up with a similar demand for compensation for damages for alleged historical training shortcomings.
I would suggest that you make sure you understand what any 'liability insurance' covers you for and for how long AND what it does not cover. I am not knocking the insurance product to which you provide a link but just make absolutely sure you know what you are getting.