I agree, but I don't think it is "calculated" in decimal places (if the makes sense). Sort of like how slide rules didn't give you "decimal precision".
Decimal places are just a convenient shorthand for describing the rough order of magnitude of the available precision. "Three decimal places" means roughly 30dB or 0.1%.
Slide rules absolutely give you decimal places. A decent slide rule might give you three decimal places of accuracy. A really good one might give you six decimal places, or 0.0001%, or 60dB. You could more precisely quantify their accuracy than just a rough order of magnitude, so perhaps the accuracy would be 55dB or 62dB, but "decimal places" gives you a sufficiently good idea of the accuracy for most purposes.
To bring it back to the digital comparison, a really great slide rule that's accurate to six decimal places is equivalent to a digital computer with 20 bits of output. If you put in a ton of work building an incredibly precise slide rule you might be able to add another order of magnitude and get seven decimal places. On the digital side, you'd only need to add 3 or 4 more bits to match that improvement.
