With the notable exception (unless it has changed recently) of Verizon, phone service is not really device-specific, in the sense that the provider needs to specifically enable access for that device or model. Providers follow standards and use specific network bands. Phones follow standards and support specific network bands. To the extent that these work with each other, the phone should work on the network, unless the network has done something specifically to disable it or disable phones they haven’t specifically certified (eg… Verizon, to my knowledge).
Phone companies might provide “official” support, in which they would consider themselves responsible for making sure the phone actually does work on their network. Without that official support, they aren’t responsible if it doesn’t work, or if you need to deal with configuration or implement workarounds to make it work.
All this is especially true for data. For calls, it can be more complicated, as there can be configuration complexities, particularly with VoLTE, which is why that is discussed quite a bit in the US users thread. This seems to be a problem with Fairphone, not US providers; LineageOS generally seems to make VoLTE work in the US, whereas it doesn’t generally work in the stock OS from Fairphone.
In my experience, both with the Fairphone 4 and other unusual devices without official support, T-mobile generally works reasonably well in the US, as does Google Fi, though the latter can have some weirdnesses sometimes that are not necessarily Fairphone-specific.
What that site is stating is that no provider officially supports the Fairphone 4 in the US (ie, will make sure it is working). However, with the exception of mmWave 5G, it lacks only one 4G band and no non-mmWave 5G bands on T-mobile, for example. Checking on a FP4 in California with a connection to T-mobile, it is currently connecting with LTE/4G band 2 (if there is any 5G service in the area, my Pixel 7 Pro doesn’t connect to it either).