Apparently a simplex is a concept in geometry. I'm guessing this is the intended meaning of the term in the name. When most decently well informed, native speakers of English hear "simplex," though, they are likely to hear "herpes simplex" and so are likely to wonder, as I did, whether, or why, the program is named after a sexually transmitted disease.

I thought it's related to the simplex method in linear programming

If you gave me the chance to come up with 500 things related to simplex, herpes would not be there.

I would never have made that link.

I for one did not make that connection. Only thing I thought of was https://simplex.chat/ (which is fortunately not marketed as a dating app)