Spitfire Audio Library Manager has been replaced. And no, you didn't overlook anything. As usual, Spitfire provided no helpful info during this installation process.
The two apps have similar names, so when I was asked if I needed to download the Spitfire app, I said "no," at first, thinking it was referring to Spitfire Audio Library Manager. But when the installation didn't work, it occurred to me that the name was slightly different, indicating a separate app. (You'd think they could have clarified this, during the process.) So, I started over, intending to download the app, but since I had previously answered that I already had the app, the web site remembered my selection—and didn't offer me the download link.
I literally had to go to another computer and start over, once again, just to be able to download the app. Then I sent the app to the intended computer via file sharing.
Spitfire has the absolute worst customer experience (among the "major" developers). And when I wrote them to ask that they improve this process for future customers, they simply replied, "We replaced the app months ago" (or something like that). So, in other words, I should have been a mind reader and should have realized that Spitfire Audio Library Manager had been replaced, and they were of course under NO obligation to merely mention that the app required to download the LABS instruments is different from Spitfire Audio Library Manager. If I didn't just "know" that already, that's my fault. They are so arrogant and dismissive.
P.S. The LABS piano sounds great, but the strings sound terrible.