I'm five days out from AVR, and your docs' explanation makes a lot of sense to me, intuitively. If that's true, once the left ventricle remodels back to its original size, HR and diastolic BP should return to lower values.
I had a relatively low resting heart rate (high 50s) before surgery, and now it's in the 90s. Makes sense that my heart has to pump extra to keep up with all that volume. My diastolic BP is in the 80s right now, so not super high. This explanation also jibes with my observation that the upper end of my heart rate is unchanged. It takes a brisk walk, even 5 days out from OHS, to get my HR to 120 right now. That change seems similar to my HR's response to exercise before surgery.
Did your diastolic BP and heart rate go back down?