If a button's label has an ampersand (&) character before a letter, that letter will be marked as a type of hotkey for that button. When the button is on the top-level of a toolbar, pressing Alt plus that letter activates the button.
In my case ampersand is used for F&older in one toolbar and for M&ove As in other toolbar.
Even if the toolbar containing M&ove As is top level a pressing of Alt+o opens (unfolds) Folder button.
Directory Opus 13.17.3 (Beta) Build 9340 x64
OS 6.1 (B:7601 P:2 T:1) SP 1.0 "Service Pack 1"
You (we) see it as top level, but nothing says that Opus is internally searching for accelerators matching your input in that order.
Objects are stored in memory in some order that we do not have our say and since, to my knowledge, nothing has been designed to achieve that, it's not implemented to do so.
Note that this could be tricky if you combine : some floating toolbars, some vertical and horizontal toolbars, some attached to panes, some not ... how would you consider how to order them.
Since accelerators configuration is at your hand, it leads back to my previous advice.
OK. I think that now I see it. The sentence does not say about button "in top-level toolbar", but about button that "is on the top-level of a toolbar".