I can see how this would be good for Twitter’s competitors, but how is it good for Twitter? They paid for something and would just give it away for free.
Twitter is currently hurting its own clientele. So essentially, it has a product that is already in the de-valued category. When you knowingly are hurting your own clientele, you are below "optimal" - it's sub-optimal.
The core component here is checks and balances. If Twitter's source were open source, then "many eyes" could keep an eye on it and prevent it from becoming odious.
The less odious the algorithms become, the greater value that would have for Twitter.
So; that is how it would be good for Twitter itself. This idea that closed up, secret, untrustworthy source codes are the pinnacle of value is a flawed proposition when those closed up, secret sources are hurting the clientele. The greater value is tied necessarily to greater trust and right now they have a LLLLOOOONNNNGGGG way to go to get back to being a trusted partner