The most important thing to remember about physical therapy is that they are your knees and you are the only one who has the right to say what will happen to them. Other people, including your surgeon, his PA and your PT therapists, can advise, but it is your right to choose whether or not to accept that advice.
CONSENT: what it means and how it can be used
Saying no to therapy - am I allowed to?
The PA who spoke to you is misinformed. It may be what she was taught, but it is wrong. There absolutely is no magic "window of opportunity".
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
Also, 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.
There's no need to work hard at exercises and pushing through the pain will only upset and irritate your new knee, making it more inflamed and swollen:
Myth busting: no pain, no gain
Go to PT by all means, but set the rules for what you will allow - no excessive exercises and no pushing or pulling on your knees.
a. If it hurts, don't do it and don't allow anyone - especially a physical therapist - to do it to you
b. If your leg swells more or gets stiffer in the 24 hours after doing it, don't do it again.
PT or exercise can be useful BUT take note of these
If you happen to have a PT therapist who will work with you and follow your wishes, hang on to her/him - those therapists are like gold dust and you can work well with them. If you have one who is insistent on you doing everything their way, on a rigid time frame, then leave.
It is possible to rehab a knee without any PT - many of us (including me) have done it. Your normal activities of daily living (ADL) are sufficient exercise.