It is very unlikely this is about having multiple accounts as if that was the case then most likely he would have been banned directly instead of being limited on the amount of money he can bet.
What is happening to him is something which is common at fiat casinos, which is that your account gets limited, basically a casino offers their services to recreational gamblers but professional gamblers are bad for their business, so instead of banning them they limit them, as such it is unlikely his account will ever be restored to its original state.
It’s true, multiple account is of the table here because bet limitation is not the punishment for committing that violation. I notice that the OP has same issue on multiple casino which means he is always in profit that’s why bookmaker limit the account on betting huge amount. I already saw this same scenario before which casino decrease the betting limit and worst ban the user on playing in the casino because he is not giving profit on the casino side. Casino has the right to do it if the user is hurting the business profitability.
I can understand why a gambler may not like this, however a casino is a business and many of them on their TOS specifically say that their services are destined for recreational gamblers only and if you are a professional gambler then you are not welcome as you bring them losses.
However if a gambler is able to somehow play with an advantage when making sport bets they have a lot of options and can use most cryptocurrency and fiat casinos or even use a bet exchanger and forget about getting limited over being a professional player