I think there may be something in it for regular bikes. These are geared higher than unicycles which is 1: 1 (or 1: 1.5? For geared unis). What I have seen is that a lot of people cycle with too low a cadence / too high gear. At least in the past when I did road cycling the rule of thumb for optimum power transfer was that on flat road (=no hills) you should have a cadence round 90 (+/- a little for individual variance) which means that you tread down three times a second, both legs included. This is atypical for most people on a bicycle while touring or commuting to / from work. They tend to have a much lower cadence / higher gear. This in turn means greater strain on the knees. In addition, they sit low with much bend in the knees, which also means more strain on the knees. On a unicycle (and while road cycling) high cadence and sitting high with minimum bend in knees is the standard (at least for me).
High cadence results in higher strain on cardiovascular system (aerobic work) and lower strain on muscles and skeleton (breath rate and hart beat rate go up) while low cadence results in higher strain on muscles/skeleton and lower strain on the cardiovascular system (anaerobic work, breath rate and heart beat rate go down).
So maybe cycling is good or bad for knees depending on how one cycle. I guess your therapists have the most experience with those who ride regular bikes and are ordinary people using their bikes for commuting and touring.
Just my thoughts.