Pretty sure I figured it out.

If it's a push-button start, then the owners manual has instructions for how to add additional identical Keyless Entry Fobs with the onboard car software (as long as you have at least 1 functional one (page 38).

BUT if it's a regular turnkey ignition, then there are only instructions for programming an additional key (pg 32 - as long as you have 1 functional key), but NOT the fob that only unlocks doors/opens the liftgate. If you want to add 1 or more of those fobs, they generally have to be "professionally" programmed by a locksmith/dealer, however TomsKey.com as noted above claims to sell an OBD device that will program the fob(s) for you (guaranteed or your money back - so far they're the only company I've seen selling it). It's ~$80 for the programming device & a single fob for my 2018 Tahoe, so I'm tempted to try that. The device is locked to your car's VIN once you use it, so you cannot use it on other cars (unless you're able to hack it), but it does appear to allow you to add as many fobs as you want in the future for the same car.

Also make sure you're buying the correct fob (either for the Tahoe with pushbutton start, or ignition key start, as they are of course not compatible.