There are a few things.
Bill C-76 prohibits foreign contributions. The problem is that third party entities receive funds from various sources, and those sources lose their character as they get commingled in the general revenues of the third party. That is one aspect. Bill C-76 prohibits contributions but does not address the commingling.
Personally, I am of the view that if third parties, specifically in the context of a short electoral period and a short pre-writ period, want to run political campaigns, they should be governed around generally the same principles as other participants in the system. This is recognizing that third parties are distinct, with a purpose other than to achieve office—in fact, they don't—but still, if they wish to participate in the campaign, fine, but they should be subject to a regime that limits their influence proportionate to the message they want carried.