What does it mean for a random number generator's sequence to be only 1-dimensionally equidistributed?
This website provides the answer:


  A xorshift* generator with an n-bit state space is $n/64$-dimensionally equidistributed: every $n/64$-tuple of consecutive 64-bit values appears exactly once in the output, except for the zero tuple (and this is the best possible for 64-bit values). A xorshift+ generator is however only $(n/64 − 1)$-dimensionally equidistributed: every $(n/64 − 1)$-tuple of consecutive 64-bit values appears exactly $2^{64}$ times in the output, except for a missing zero tuple.


In the case of a xorshift+ which uses 128 bits of state, n/64 - 1 = 128/64 - 1 = 2 - 1 = 1, so the Wikipedia article was correct in stating that that particular generator is only 1-dimensionally equidistributed.