That blanket statement is either not true or incredibly sad: certain ThinkPads might have a great Linux experience (I still don't know which exactly, I am guessing the ones certified by Ubuntu should be alright), but random ThinkPads (even trusty T-series, nevermind random hipster tablets and cheap models branded with the same name) still have major issues like suspend/hibernate and dealing with external displays and docking stations. Then that "smooth" Linux experience includes the common issues, like dealing with Bluetooth, that are common to all manufacturers and distributions.

Do your research before deciding on an exact model.

The Dell XPS 13 has slightly better support (for Ubuntu at least) and has official documentation on their website. You can even buy one with Ubuntu preinstalled.

I had to tweak one or two bios options for my X1 6th gen, the XPS 13 worked flawlessly out of the box.

Dell XPS laptops can come factory-installed with Ubuntu, with official support.