A sudden announcement like this from Twitch doesn't well indication for the gambling company. It is believed that Twitch may have banned this gambling sites due to some kind of restrictions by the government or others. Nowadays gambling has become a big industry. But I hope that such restrictions may be reduce in the near future. Besides, all these companies have to take alternative route for gambling promotion.
However, gambling companies have to follow Twitch's rules and I'm sure they will be able to find other ways to help them keep promoting. The restriction may be temporary but it will not stop gambling sites from promoting because they have already benefited a lot from the promotion. So even if Twitch has banned the promotion of gambling, the casino can still do promotion elsewhere and that's not a big problem for the casino.