[ZeroGravitas]

Genuine question, what does that line mean?

Is it saying, the UK will have to leave the EU (for whatever definition of "leave") if that side of the vote wins.

Is it saying that the UK cannot change its mind ever i.e. it cannot rejoin even after a change of governtment and a second referendum?.

Or is it just saying, the result of the vote is the result?

[namelost]

It was a government policy, not a law. It is basically meaningless.