This reminded me to read that thread again. If you sort comments by "NEW", you'll get the most recent (and thus more accurate) results.

So, I tried it. Took 10 minutes, most of which was waiting for back and forth responses (reps are handling multiple chats at a time).
This is how it went:

Did it yesterday via the in-app chat, went like this: 10 minutes, no arguing needed.

Me: Hello, I would like to do a SIM swap on my xxx-yyy-zzzz line.

EID is 890400000999988887777666655554444.

TMO rep: Sure, I can definitely help you out to activate your eSIM. just a heads, T-Mobile postpaid does not support the eSIM. Would you still like to proceed?

Me: As in, does not support, but can be done, and I accept the responsibility and know there may be issues?

Her: Thank you so much for your understanding, /me. There might be glitches, but no worries, I can activate it for you,

Her: (Warning about contacts stored on SIM / downloaded visual voicemail deleted / Should take minutes, but may take up to two hours)

Me: Yes, I would like to proceed.

Her: Got it! Doing it now! (sends PIN)

Me: (Sends PIN back)

Her: Thanks! Almost done! 2 minutes please.

Her (two minute later): Done with the process!

Instead of steps 5-7 from the reddit post, I Scanned the QR linked in multiple comments in the reddit post ( https://imgur.com/a/3G0fMZc ). Done.

I used the mobile App chat, with my physical SIM in an old iPhone SE, on WiFi, so I was able to continue through the whole process, and go on to troubleshoot a related issue with the same rep in the same chat.

I did the EID via copy/paste to avoid any typos. iCloud cross-device clipboard came in handy here.

Despite having no physical SIM in the iPhone XS during this process, the eSIM line shows as secondary.

Heads up for anyone with an Apple Watch on cellular: Watch lost connection as soon as it went through. I had to unpair and repair it. TMO rep was kind enough to check on her end, and stayed on chat to make sure things worked.