So what happens when you press customize? From looking at a relevant changeset[1], it seems that only Allow enables the ads (setting
suggest.quicksuggest.sponsored), while Customize does not (and the comments specifically says the user remains opted out). The related bug is marked as confidential[2].
Incidentally, here[3] is an interesting list of settings relating to the address bar that may or may not be documented elsewhere.
[1] https://hg.mozilla.org/releases/mozilla-beta/rev/6348c24c98c...
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=1693126 I may be remembering this wrong, but most bugs on bugzilla are open to the public, this one isn't, presumably because they weren't interested in public discussion
[2] https://hg.mozilla.org/integration/autoland/rev/d3de5eaaaa15