What do per-year healthcare costs have to do with an underfunded pension?

Was one of the benefits of the pension a healthcare plan?

The article doesn't say that it was.

If it was, though, that is pants-on-head stupid. A pension is supposed to be a fixed payment investment, not a variable-payment one.

They're separate benefits and don't have to do with one another. Retiree healthcare is a rare benefit in the non government world, and is usually classified as OPEB (other post employment benefit).