Yes, if you are injecting CO2 (not Excel though), then an airstone will decrease your CO2 levels.
Here's how it works.
If you have a tank with nothing in it, then it will have a certain CO2 level, somewhere between 2 and 7 ppm. This is what we will call the 'equilibrium level' of CO2, or the level that the water tank will naturally want to return to if left alone long enough. Increasing surface agitation, whether from increased flow or an airstone, allows for gas exchange to occur much more easily than with a still surface. This gas exchange makes the tank want to return to its equilibrium level of, say, 5 ppm. Now if we have plants in the tank that are actively using CO2, then our CO2 levels are going to be reduced below the natural state of 5 ppm to lets say 1 ppm. In this case, when surface agitation occurs and attempts to restore the natural level, we are actually introducing CO2 to the tank.
Now, if we are injecting CO2, we are raising the CO2 levels ABOVE their natural levels, up to say 30 ppm. In this case, when we cause surface agitation, the natural state of 5 ppm that we are attempting to restore is LESS than our current levels, which causes a net LOSS of CO2.
This isn't to say that we can't have any surface disturbance when injecting CO2. Lets say that I'm injecting CO2 into my tank to bring my tank up to a maximum of 30 ppm, but my HOB filter is causing enough agitation to cause my levels to be less than that, say 20 ppm. So my CO2 is still elevated, but it's not as high as it could be. This is one of the reasons that pressurized CO2 is plain better than DIY CO2. DIY CO2 has a constant maximum CO2 level that can't be easily increased. So if you DIY CO2 puts out 25 ppm of CO2 but your HOB is killing 10 ppm of that and your bubbler is reducing it another 15 ppm, then your CO2 levels are going to end up being just a little more than the natural 5 ppm. If you instead have pressurized CO2, you can easily increase your CO2 output to a maximum of, say, 55 ppm which is reduced by 15 ppm by the bubbler and 10 ppm by the HOB to a healthy 30 ppm level. Of course, if you got a filter that doesn't disrupt the surface like a HOB and didn't use an airstone, you could still run at 55 ppm, of you can turn it and run at 30 ppm.