I'd have to see the actual pension plan of course to assess the merit of both sides claims but having done contracts the root cause can easily be anywhere from being to malignant. Given how it appears CBAs are negotiated I can imagine this was agreed in principle but not fleshed out very well.
Strictly speaking from a business standpoint, once you have language, you go figure out how to warp it in your favor. This is why doing contracts well takes so much diligence. I also tell those involved "we're not writing this for good folks like us. We're writing the language for possible bad actors enforcing it after we're all gone".
Why would folks do that? Because they can and have low chance of being held accountable.
That last statement speaks volumes on their level of arrogance, entitlement, and base view of the humans involved in their industry.
No wonder we may not start or even have a 2027 MLB season.