How could someone prove something like this?
You asked for the txid and he gave it : he proved his point! That's all.

Above all, it is clear the dev has still lots of coins : would he be a scammer he would have stayed a little longer to finish selling and win a few btc more.
Leaving make its remaining coins loosing value quickly.
Either something happened to him or at least to his internet connection, either he is not acting rational.