They don't want it to to heal in place and leave us with no ROM.
That's a pack of rubbish they've told you there. It doesn't even make sense, no matter how I look at it.
Your knee will
not set in one place if they leave it alone, and if you only exercise it gently.
There's no need to rush to get ROM (Range of Motion) because it can continue to improve for a year, or even much longer, after a knee replacement. There isn't any deadline you have to meet:
Myth busting: the "window of opportunity" in TKR
The concept of a window of opportunity is just an old urban myth that should have died long ago.
For the first month after surgery, your knee needs time and gentle treatment, so it can start to heal. There's no good reason why it should be stuck in one place if it's not manipulated. Right now, it's swelling that is stopping your knee from bending more. As the swelling goes down, your knee will naturally start to bend more. Pushing on your knee and forcing it to bend will just make it more angry and swollen, so it's counter-productive.
It's not exercising that gets you your ROM - it's time. Time to recover, time for swelling and pain to settle, and time to heal. Your ROM is there right from the start, just waiting for all that to happen, so it can show itself.
My surgeon doesn't allow any formal PT
at all for the first month after a knee replacement. He says your knee needs that time, to start on its journey of healing. For that month, we rest, ice and elevate our leg, and walk around the house. The walking is our exercise - and walking is very good exercise.
After that month, we just go to PT once every 2 weeks, where we are shown a few new exercises to do at home.
His patients all do well and achieve good ROM, as I did, and he hasn't had to do a manipulation to help with ROM for the past 4 years. I think that speaks for itself.