I just asked T-Mobile and they told me this...
T-Mobile Twitter: Thank you so much for filling that out, and for your patience as I did a bit of digging on this! The Note20 Ultra 5G doesn't have access to our stand alone 5G network at this point in time, so there is no need for a new SIM card at this point in time

If anything changes with that, you would be notified by a text that you'd want that new SIM to get the best 5G connection
Me: So wait the Note 20 Ultra 5G doesn't have 5G on T-Mobiles Network?
T-Mobile Twitter: Oh man! I'm so sorry for the confusion, Alex, it definitely does have 5G! Phew, sorry for the scare that I just gave you there haha - let me explain what that meant by the stand alone 5G network.
As we roll out 5G, we have a few different components. The first, which makes up our nationwide 5G footprint, is what's called "Non-standalone 5G". This relies on mid-band LTE coverage in order to access 5G, and is what your phone (and most 5G devices) has access to, as well as the most widespread 5G signal we have available.
Recently, we have begun rolling out stand alone 5G as well, which does not require LTE signal in order to receive 5G. With SA, the device connects to the 5G radio and core network. This type of 5G connection does require a new SIM, but isn't available nationwide yet, and is not accessible on all 5G devices. I hope that helps to clear up some of that confusion!