The pagans never used their version of tongues for private use. It was always used as part of their temple ritual.
Voodoo doctors use tongues privately. So do the occult.
Isaiah 8:19 And when they shall say unto you, Seek unto them that have familiar spirits, and unto wizards that peep, and that mutter: should not a people seek unto their God? for the living to the dead?
The sticking point, the question that is difficult to get around is that Paul says he speaks with tongues more than them all, but in the church he would rather speak clear language so that others can understand him. It is clear that he did most of his speaking in tongues away from the church. So, does this mean that Paul, speaking in tongues by himself and not among the other believers in church, was speaking pagan tongues? Because this is what you are saying - that anyone who speaks in tongues by himself and not among other believers for the purpose of interpretation, including Paul, is speaking pagan tongues. Therefore, are you saying that Paul is engaging in pagan worship when by himself, but in Christian worship with other believers?
You see, the phrase "but in the church" is the large elephant in the room here.
The sticking point is Paul was testifying to something they all knew because he was speaking in tongues more than them all in church. Paul cannot testify to that if he was referring to private use of tongues not done in the assembly.
2 Corinthians 13:1This is the third time I am coming to you. In the mouth of two or three witnesses shall every word be established.
So the but is about Paul preferring to speak five words understood by all rather than a thousand in tongues.