It seems to me that this article is wrong or misleading in claiming that every object moves through spacetime at the same speed. With respect to its own proper time, an object that is moving faster through space is also moving faster through external time, since its proper time changes slower with respect to external time; so overall the object must be moving faster through spacetime. Something in the vicinity is true, i.e., it works if you make the time coordinate imaginary (or do something similar -- see, e.g., here), but this does not seem to be the commonsensical definition of moving through spacetime. The Wikipedia article on four-velocity, which is cited by the article above, also does not seem to mention this aspect and, at least to me, got confusing in the magnitude discussion. Am I missing something?