No, he’s saying that it may be more because of the added amount of texture samples used.
Why?
Because the paint layer is handled internally like everything else - with textures.
So paiting 6 layers with 3 textures each on a tile will consume 6×3+ potentially 6 samples (one per layer worse case scenario).
Whereas an auto material based on normal slope won’t consume the extra texture samples at all.
What you have to do is test the load on the auto material from the slope sampling. The operations can be more costly than a texture sample.
They can also be optimized though.