Hi there,
Indeed, Leave it to marketing to get a little “Loose” with the BMS term, LOL…it’s not really, more of a glorified Battery charger.
The PMIC’s they make (Nordic) are the truth. IMO all the (2 pins) used to do the weakest battery monitoring I’ve seen to date is this half measure, using the GPIO’s I say just do the PMIC (i2c) and 1 GPIO for interrupt you gain and save at the same time. SMH but size matters in XIAO!
also worth noting the MAX versions are good too, They have them or similar in the Wio_terminal and the external Battery Pack, Now that is a BMS light!
HTH
GL 
PJ 
BTW, Nordic sent me the PMIC dev kit, I think I posted the pics here somewhere… It comes with every possible setup one would need to fully evaluate the PMIC and the Awesome accessory it is.
the Seeedineers would do well to get one and give it a go. 