You're missing the point. They don't have to 'sanction a plan to fund a stadium for a non-existent team'. They would build an Olympic Stadium because a winnigng bid would need one, period. Are you telling me Ottawa and Queens Park are going to dictate the design elements? It's the COC that would do most of this, while the city and a potential long term tenant may influence the design in terms of form factor. The COC either follows their own drum, or if it's required in order to actually get the thing built, they work with whoever is pulling the strings. These parties can influence the design, form factor and potential upgrade capability - but not the dollars spent by government since they would already be spending the money.
I'm not saying his reasons for saying no go any further than "we can't afford it" but by having done so, he actually holds more of a hammer than he did before. The COC would have no choice but to come to some agreements in order to get everyone on board, and influence in the stadium design doesn't affect their bottom line at all.