A mistake on Infocom's part? To include TI Pro with IBM compatibles?
See other cover: http://www.mobygames.com/game/zork-the-great-underground-empire/cover-art/gameCoverId,28778/
Check the part that lists other systems: "Use the IBM PC version for your Compaq and the MS-DOS 2.0 version for your Wang, Mindset, Data General System 10, GRiD and many others"
, while the TI Professional & DEC Rainbow are mentioned separately
Now, that second cover
you found probably shouldn't be filed under DOS?
I'm not familiar with DEC Rainbow, however it had separate order code at Infocom - check that listing I linked in previous post and our cover scan saying DE2 (also: RX50 disk), not IB2.
RX50 disk drives used specific 5.25" SSQD diskette format, I'm not sure if 1980s IBMs could read it? Today we can do it: http://home.claranet.nl/users/pb0aia/vax/rx50.html
Please note how this Infocom history text file gives separate dates for introduction of their interpreter to IBM PC, TI Professional, DEC Rainbow, MS-DOS.
This also means something.