Product ASIN: B005REEWYY tells us that it is manufactured for lithium HV (LiHV) batteries. Therefore its termination voltage logic will stop charging of some inserted battery at 4.35V.
Firstly, such intelligent external power supply will not bring Librem 5 battery to explode while … Purism thought up front about this (BPP-L503 battery probably having BMS integrated within it, making it to be the one of PROTECTED type – just my unofficial guess).
Secondly, many internal components of the Librem 5 are to be provided with 3.3V and engineers that made this product (with add-on components like M.2 cards) allowing max. input voltage of 4.20V, so there should not exist any reason to insert some 1S LiHV that fully charged provides +/− output of 4.35V (or even 4.4V).
@frank, please refer to this post: External battery charger for Librem5 - #88 by fsflover, and to few surrounding ones.
Only universal battery charger, known to me that here related battery fits into it, and that will terminate (although just partially, and therefore connected battery should not be left there for extended period of time) charging at 4.2V is this one: https://www.aliexpress.com/item/33059035109.html.
P.S. Just putting few simple thoughts here that are to be followed, as nothing here needs some expert explanation why is necessary to differentiate 1S lithium-ion 3.7V batteries from 3.8V batteries, etc.