Is it true that it is easier to obtain a PhD in Europe than the US on average?
Short answer: No.
Long answer: Determining the minimum bar for which a PhD can be awarded is a bad exercise. If you are merely working towards the minimum you will not successfully complete a PhD.

One thing to note is that PhDs in the US tend to be open ended (to the extent you can continue to get funding) while in Europe they tend to have fixed durations. This means in Europe the bar comes whether you are ready or not while in the US you get to chose when you  defend. Neither is easier, just different.