Natural convection isn't a strong force.
Picture a campfire on a still night. You can stand next to it and most of the heat is going straight up. Then a gust of wind comes along, and immediately you feel the heat blowing towards you and you have to step back to not get burnt. It doesn't take much to move heat around.
As the air is already flowing through the solar collector downwards, there is essentially no "force of convection" to fight against.
If you want to convince yourself:
You should find that there is no significant difference in the CFM in both cases because the "force of natural convection" is very weak.
You might also find that you get more CFM on that sunny day with a hot collector due to the affect of gravity pulling the cooler, more dense inlet air (from the near the ceiling) down through the collector (before it gets heated within the collector).