[yk]

The article is correct. The nit from grandparent is, that it uses some outdated language. So if you do not look to closely in special relativity, you will find that the mass quite often appears together with the Lorentz factor, which describes time dilation. This lead historically to the claim that moving objects are heavier than the same object at rest. But since this does not hold in general relativity, it is nowadays usually assumed that mass is always the rest mass. And so the article reads a bit outdated.