The 'official' line, as in this leaflet from the musculoskeletal physiotherapy team at Oxford Health, would appear to be not so much that you shouldn't use a walking stick but that it's important to use one that's correct for your height
https://www.oxfordhealth.nhs.uk/wp-content/uploads/2014/08/OP-103.15-Safety-information-leaflet-walking-sticks.pdf
So it might be best to push your physiotherapist towards ensuring that you get a suitable one.
I suspect you know what might really help best though and that's to lose some weight. (Yes, I know that it's FAR easier said than done!). I've got a friend who's a 71-year-old, severely overweight Scot. He's been struggling with knee problems for years, including seeking help both from the NHS and privately, as well as regularly attending a gym for suitable exercises. Recently though, after many years of good intentions but little else, he's finally managed to lose a couple of stone and he's seen a BIG improvement in his walking ability.