I have no doubt that 75% of Waitrose customers do some sort of online grocery shopping, but I have serious doubts that their poll was representative of the whole of the UK.The original poll was apparently representative, as others have pointed out, but in any case, would that really be so surprising?
Over-60s have been advised to stay at home entirely for much of this year. Many working age adults have also been working from home mostly or entirely for several months having either never done so before or done so only rarely. Lots of people have been sitting around very bored with plenty of time to do online shopping.
Meanwhile, there were queues outside physical supermarkets that could last for an hour or more if you turned up at the wrong time, often starting from early in the morning well before a store actually opened its doors during the early panic-buying phase. Today, happily, the worst of the queues seem to be a fading memory, but where did all those people go? They presumably haven't stopped needing bread and milk and toilet rolls. And you can still have to wait to go in at relatively busy times, and then it's not a particularly pleasant experience trying to keep a reasonable distance from everyone and wondering who's previously handled whatever you're about to buy off the shelves.
It seems entirely plausible to me that 75% of all supermarket customers do at least some online shopping now. Frankly, I'm surprised it's so low under the circumstances.