Hi pseudonym thank you for taking the time to report this bug!
I have not been able to reproduce this bug on 1.3.18 (6). I have been able to install extensions from the Chrome web store, whether in Lockdown mode or not.
However what you show in the video looks like a different bug I've just fixed. The bug would not let a user install any chrom extension, and not show any error to the end user. This is due to the "Chrome Extensions" setting. A fix for this is on the way, but not yet published.
In the meantime, could you double check that "Chrome Extensions" are enabled under "Settings → Advanced"?
Thank you for your help in making Orion a better browser!