This is talking about if the plan cannot pay it's pensions. That's why it starts with:

The Government of Canada has a legal obligation to pay plan member pension benefits.

It's not talking about if actuaries predict a future funding shortfall. In that case, they can change the rules before there's actually a shortfall, as they did in 2012.

I don’t dispute that they’ve renegotiated contribution rules, I don’t know the history of this pension fund that well. Typically these rules are renegotiated with union agreement.

The unions have no input into contribution rules. Any changes are decided unilaterally by the government, as shown in your source (look for "Jobs and Growth Act, 2012").

EDIT You are correct, that the government guarantees the fund if it completely fails. What I meant to say is the government isn't liable to top it up if it's underfunded. My bad on the wording.