Maybe Verizon is not going to cooperate with Purism. It’s not like Purism is big enough to heavy Verizon into mending its ways.
More accurately, Verizon supports (allows) the modem in the Pinephone (Quectel, soldered in, I believe).
Now if only Quectel made a modem that fits in the Librem 5 (and meets any other relevant requirements) …
As always there are three parties (carrier, modem manufacturer, phone manufacturer) and the stars have to come into alignment for it to work.