I'd say both are about the same. The Librem 5 physically puts the Modem and Wifi/BT modules on a physical card, where the PinePhone make it all into a single board. I believe both have the same physical/logical seperation (i.e. Modem and Wifi/BT are on a Bus versus integrated directly into the CPU), and both support a hardware disable due to the physical seperation.

Both Phones have to load firmware into the Modem and WiFi/BT.

For all practical purposes, it looks to me that there's no difference between soldered and slotted when they both have a physical switch to disable them.

In theory you can change the modem/wifi in the Librem, that may be more "open" but you pay for it in size.

the librem isn't more open than the PinePhone at all. The only difference is that they put the modem on a separate card… but that doesn't make it any more open at all. What matters, given the fact that there just isn't any open modem out there, is the fact that the modem is kept isolated from the SoC in a way that doesn't allow the modem to access anything the SoC doesn't explicitly give it.