This is correct.
When I did a few from bamboo, that material ended up being more like 3.5mm+ so the internal stack worked out ok (at 10.5mm)
I think Ponoko has some 3.2mm material but I’m not sure if that’s totally accurate or just “advertised”. I’ve found some cast acrylic can vary up to like 0.5mm on the same sheet, so I usually have to see what I end up with.