Is it legal (or at least not soecifically illegal) to go to this park at night? It lists as closed, but so does Central Park and people are walking and running there 24/7 without any trouble from the police.

Technically, you may be issued a civil summons but these cases are very rare - https://www.nycgovparks.org/rules/section-1-07 . Years ago in Brooklyn someone asked me about police issuing them a summons. In Central Park I heard of people being to leave because the park was closed. No guarantees but highly unlikely.

If u are there just walking or running & causing no trouble, u should be alright. U might get a warning from the police, u never know!!