Yes, the commissioner can pick solo without sharing the draftboard with anyone for live in-person drafts.
This can be a good workaround if you have already drafted on another platform, but want to play out the rest of your season on Sleeper to take advantage of many of our awesome in-season features.
For example, if you already drafted on Yahoo! or ESPN, or want to do an auction draft (which we don't currently offer), but still want to leverage Sleeper for the rest of your season, just ask your commissioner to input the picks into the draft board and you can run the rest of your season on Sleeper. The whole process takes 5-10 minutes at most.
This can also come in handy if you do slow-drafts via email, and want the commissioner to handle the input on the league's behalf.