Yes, the docs aren't updated. Taskbar3's settings' names tend to change from version to version too. See the docs for v0.303 and you'll see stuff which isn't documented in the later versions.

Doesn't using TaskbarMaxButtonCount make taskbar3 stop accounting for tasks after it hits this value?..making scrolling not doable.

For all I know, you have to autosize taskbar3, if you want to maintain a consistent task button size when doing vertical tasks yourself. I posted some guesswork code on the second of the above forum links that does this--one column of same-sized task buttons. It intends to autosize taskbar3 proportionately to the number of tasks, and set the number of lines (!TaskbarLines) to the number of tasks. The third link above is to Smurth's theme, which by judging from the screenshot, already does the same.

Of course this means you have to have taskbar3 boxed somehow to create the illusion of blank areas where task buttons otherwise displace. So the lsbox (or possibly SkinBox [I haven't used it before]) would be big enough to cover the four icons.

My thoughts for this involves that you autosize the tasks, and whenever there's more than four tasks, then you allow buttons or whatever for scrolling (first link above). How that scrolling systray code works is beyond me; I never thought you could go 'out of the box' without modules 'wrapping', since that is the case whenever I tried doing so (but that was rare and short-lived experimentation).