Banktoturn
Yes, some definitions should be clarified so we are able to understand each other…

Let's try:
The airfoil thickness as well as the camber, are normally expressed as % of the chord, right?
So, let's assume that an airfoil without camber (e.g. a flat plate) that has 3% thickness is considered thin.
If the wing has a chord of nine inches, the absolute thickness should in this case be about 1/4 inch.
According to the chart above, a thin flat plate is best at Re's <5000, so the actual wing chord at such a low Re is probably half the value mentioned above, so the absolute thickness would get down to 1/8 inch.

A cambered airfoil doesn't necessary have to be thicker than a flat plate, but if you have a camber of say 4%, the airfoil thickness may be more or less higher depending on what sort of airfoil, wing structure and covering material used.
E.g. indoors often use thin covering film over ribs between the spars, which make the absolute camber value almost equal to the absolute thickness.

As for the flat plate being the best airfoil…, I realise that you still are sceptical, but think that it applies only at very low Re's <5,000 where the airfoil's thickness starts getting closer to the size of the air molecules….

The picture below is just to ilustrate some terms despite you may already know them well..