Following those two simple ideas.............

One of the factors is the force the airflow can exert on the wing structure. The wing structure can include the shape and strength of the aileron or it can not include the shape and strength of the aileron. If the aileron is loosely attached, it would not contribute strength would it. It would also not transfer the aero forces it's experiencing into the wing structure would it.

If the aileron were experiencing forces and transferred them into the wing, the sum of forces the wing is experiencing would increase would they not. So if the aileron is more firmly attached to a wing it loosely attached to, you would expect that wing to feel increased forces. Seal the aileron from tip to tip to the TE of the wing with more hinges or gap covering and you've more firmly attached it, haven't you.

And in effect, you've just done something else to that wing, haven't you. You've just changed it's chord. You've gone from a wing with say 8" chord, to a wing with a 10" chord (for example).

It's well understood that increasing the lever arm of a twisting force gives it more "twisting force".

We've got a wing that was strong enough not to flutter when it was 8" wide. The aerodynamic forces weren't strong enough to flutter that TE when it was back at 8". But now that TE is back at 10" and the aero forces have 20% more leverage, don't they. Not only that, but they've had an extra distance to build up even more force.

Seal the gap between an aileron and it's wing and you've just changed two systems into one. The flutter speed of two systems is going to be different than the flutter speed of one system. Which way will the speed change. Danged if many humans or computers can figure it out much less predict it off the top of their heads. And the strength systems have changed for the wing. And the force systems have changed for the aero forces.

And ain't nothing changed in nature about flutter. It's gonna do what it's gonna do and when it wants to do it. But now that you've sealed the aileron to the wing more solidly, and given the aero forces more leverage to exert on that wing, I'd hazard a guess that it's maybe more probable that the flutter can occur at a slower speed.

.... but, then again.........