Apple document lists a separate "XENIX Software Development Environment" as its own product, and the "Lex" product as a word processor. It could be the document's wrong; Wikipedia says that Yacc's first full spec was first published in '75 so it very well could be that this UniPlus Lex is an implementation of that. Hard to say without having the software...
I assume your IDOL and Thoroughbred disks are Sony 3.5" disks?
Although it's possible, I'd be surprised if lex wasn't included in both Xenix and UniPlus as it's pretty standard.
Yes, IDOL/Thoroghbred disks are all 3.5" floppies, they came with a Lisa that had a Widget, and Tecmar Quad Port Serial card installed, and a batch of also deteriorating bad Xenix floppies. I'm pretty sure I imaged them with CopyIIMac to dc42 and set the ignore errors flag, but pretty much every single floppy had many bad sectors. (The Widget on this drive had MacWorks installed and Lotus Jazz, which is the one that's on Macintosh Garden now.)
One thing that's missing from the Lisa and hasn't ever surfaced was LisaPaint. It ofc became MacPaint, but I don't think it was ever released. I'd love it if it made its way to bitsavers. Possibly it was buggy or whatever, and then just got ported to the Mac.
Inversely, there was a MacDraw (as well as MacTerm, MacProject, MacWrite.) I don't think that Calc, Graph, List, were published for the Mac, likely because they would compete with MicroSoft Word, and Jazz, and other third party products, and drive away developers, or something.)