"P may collapse" vs. Time hierarchy theorem https://en.wikipedia.org/wiki/P_versus_NP_problem states:


  If graph isomorphism is NP-complete, the polynomial time hierarchy
  collapses to its second level.


They further state that this may be unlikely, but at least it's a possibility. However, https://en.wikipedia.org/wiki/Time_hierarchy_theorem states:


  The theorem also guarantees that there are problems in P requiring arbitrarily large exponents to solve; in other words, P does not collapse to DTIME($n^k$) for any fixed k. For example, there are problems solvable in $n^{5000}$ time but not $n^{4999}$ time.


Why don't the above two quotes contradict each other?
The polynomial hierarchy is not $\mathsf{DTIME}(n^k)$ for various $k$, but rather a polynomial time version of the arithmetical hierarchy.