https://bitcoinist.com/bitcoin-otc-trading-volume-accumulating/
The evidence is very weak.
They only compared Coinbase to GBTC over two different periods. And because they saw a 20% increase in Coinbase volume "during OTC market hours" they are bizarrely attributing that to OTC market growth. They also note that GBTC volume has been in decline. They don't have any numbers to support their theory.
The remarks about "exponential growth in OTC volumes" from HodlHodl's CEO stem from the fact that their exchange launched literally last month. Exponential growth is very unimpressive with that in mind. He obviously provided no volume numbers because if he had, they'd be laughable. It's a P2P non-custodial exchange that launched a month ago...