I can confirm what Adrian Brugger wrote -- you do need to copy the BIOS bin file to a directory such as /tmp and then run cd /tmp in your command order before executing the hptc-bios-flash command from the /tmp directory (HPDM 4.7; t620; ThinPro5.2).
I found that upgrading from 0208 directly to 0211 was successful, but falling back to an earlier BIOS version is trickier. You can upgrade the BIOS several versions, but I was unable to run either the hptc-bios-flash (ThinPro 5.2) or the ami-flash (SmartZero Core 4.4) to revert back to 0208 from 0211. Instead, I was only successful in reverting back one version to L40_0210. Otherwise I got an error message like "Incorrect BIOS for your system".