...instead of one Add-ins ribbon tab, in AutoCAD® Architecture 2018. I found this Autodesk Knowledge Network article on the topic, indicating it was for AutoCAD® Map 3D, 2014/2015, but also applying to AutoCAD® and other verticals, for the 2014, 2015, 2016 and 2017 releases. The instructions worked just fine in AutoCAD Architecture 2018.
As noted, I did not need to do Steps 2, 3 and 4, but I did check to see that was all in place prior to proceeding with the balance of the steps. After unloading the partial customization files for the two plug-ins that I had installed and then closing and restarting the program, the individual panels for the plug-ins showed up on a single Add-ins tab. I did have to turn on the display of the Add-ins ribbon tab, but that was easily done by right-clicking on a blank area of the ribbon and choosing Show Tabs > Add-ins from the context menu.
Oddly, it all came in on one Add-ins tab for AutoCAD® MEP 2018, on the same machine.