Yes, I've set this up before and also installed the app in a fresh profile and in the setup process I still have the option to link to an existing device. I've basically taken the exact same steps as in the tutorial you linked and on the screen where you're asked to enter your phone number I do have the option Link to existing device in the bottom left corner.
I'm on version v7.44.2-1-FOSS (155001).
Have you used the non-FOSS version of the application as instructed in the article? Maybe try the FOSS version, it's fully compatible. If you're interested in the exact differences, see this part in Molly's readme. Although the location part is a bit outdated, nowadays Molly-FOSS sends both an OSM and a GMaps link if you share a location.