I believe he said he wanted them to "participate", which I would interpret as helping with the Supercharger rollout - i.e. deploying stations.
And they can still do that. Adding vehicles to the network adds demand...and presumably a corollary increase in supply. The fee paid by GM would simply be on a per vehicle basis to cover maintenance, electricity, and future build out. Especially with a low volume model, it makes a lot of sense for GM to seriously consider being the first to buy in to the Tesla Supercharging network instead of going it on their own...or worse, not developing any charging network. That is of course if GM is actually serious about becoming a player in the EV market...