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.