Yes, it definitely reduced CO2 outgassing, it also, more importantly, reduces all gas exchange, so O2 is less also.
I've noticed this a fair amount on my 180 gal tank.
If I leave the lids on the tank, I can get a similar case even without any scum.
When the O2 production from plants is high, there's less scum as well.
You can see how much reduction the water surface moves without scum layers.
The reduction in flow of surface turnover is very high even with high flow rates.
I have zero movement or without the scum, I get about 20 cm a second on the surface. That's a huge difference. It is also measurable and measurable as far as the night time degassing.
The tank never dips below 25ppm of CO2 at night with the scum layer, without, 15ppm or so. O2, the min max ranges are higher with the scum, without, much less extreme. I also noted that with the lids on, I had much less scum layer formation.