Hi there,
WoW, That’s sound like malarkey, I have used an 18650 for two years with a Xiao Nrf52840 BLE and The Sense variant. Depending on The BSP you use It is possible to monitor the Battery Voltage, USB in or NOT? Change the Charge Current from 100 ma. to 50ma also.
Why would anyone connect 9V to a 3.3 v max device? Smoke Test is all you’ll Get
Works with the Xiao , works with the Dev board and Also works with the Grove Expansion board with the Extra Flash installed. If you look at the schematics and Data-Sheet of theses you 'll see the trend with the PMIC’s
HTH
GL
PJ
PS, if you try and use a USB power bank Eliminates the battery soldering(I use one with Solar) Some will shut off because when the Xiao is ASleep draws so little current it thinks it’s Off. 
and powers down.