>"This actually…doesn’t not make sense"

I don't understand what you are referring to when you say "this"; what is "this"? Are you referring to a specific statement by the parent poster, or something described in the OP?

I also don't understand whether the double-negative is intentional (and meant to cancel itself out, and become a positive), or if it is a colloquialism.

  This = 'the comment being replied to'.


  This actually !(not make sense).

  V
 
  This actually does make sense.

The reason they wrote it the way they did was to convey that (not making sense) was what they expected from GP, but that instead it did make sense.