Well, that's not the context that exists relative to the question asked. I answered a question about how to access uBlock Origin to turn if off for a site or page, which means you need to be on the site/page for which you are attempting to turn it off. I was focusing on that question as one about "how to access" rather than "how to use" as well.
The instructions are context specific, not generic. But ALT will throw focus to the Chrome menu button if you are on any webpage with focus. If you're in the omnibar then you're in text-entry mode, where ALT does nothing in context.
Brian - Windows 10 Pro, 64-Bit, Version 1809, Build 17763
Puritanism: The haunting fear that someone, somewhere, may be happy.
~ H.L. Mencken