The BSP provided by Seeed does not include the DeepPowerDowe() function, which causes a compile error. flashTransport.runCommand(0xB9) can be used instead of DeepPowerDowe() to compile. See the sketch below.
sketch_apr15a.zip (1.9 KB)
You may be able to avoid the compile error by replacing the BSP with one downloaded from Github (I have not tried).
The following links may be helpful to you. Please try the attached sketch in post 1/3.
‘Sleep Current of XIAO nRF52840, Deep Sleep vs. Light Sleep’