Using proprietary software in their code base, they made billions and dominated ther market, crushing all competitors. Right now they charge people for access to the code.

On the other hand, they can't freely release the code, something which is of little benefit to them anyway. How is that a "strong case" against?

Because the parent poster suggested they in fact wanted to open source more stuff but couldn't for these reasons. Therefore, it is a strong case against.

Maybe we're using different definitions of "strong case", I'm imagining it in a court of law, a case for going one way is "billions of dollars and market dominance" - that's a strong case. The case for changing direction is "in 30 years, giving your stuff away will be less legal trouble". That's true, but it's not a strong case for changing direction, it's a minor shrug. Strong meaning enough to overturn the competing arguments and push a different decision, not just "isn't wrong".

Sure, but at the time they were creating the mess they were actively opposed to open-source software.

only if you believe that future open sourcing of your code base would bring greater benefit than the proprietary software itself, which seems very unlikely.