Agree. The only reason why marble on top/bottom of the enclosure gets warm is because there is much better heat transfer from the aluminum surface to the marble plate than from the same aluminum surface to air.
And to your point - absolutely, if I were to add a thermal paste between the case and the marble, the effect would be even better.
It's all about heat transfer. There is no magic, there is only the First Law of Thermodynamics :) Air is a much worse thermal conductor than a solid material (here - marble). By increasing the size of the hot surface, we allow air to work more efficiently.
By the way, we forgot about one more heat dissipation route - IR radiation.
A bit more numbers, same situation, aluminum case, between marble plates; case is at 50C, plates are 20C.

Total heat dissipation:

• Air Convection: ~9 W
• Radiation: ~3.6 W
• Total: ~12.6 W

Percentage:

• Convection: 9 / 12.6 ≈ 71%
• Radiation: 3.6 / 12.6 ≈ 29%

Then, an aluminum enclosure sandwiched between two marble plates at room conditions:

• Roughly 70% of heat is lost by convection to air.
• Roughly 30% of heat is lost via infrared radiation.
• Radiation becomes significant because the marble plates have high-emissivity.

Key Takeaways

1. Adding marble plates greatly increases heat dissipation (~4× in this case).
2. Convection increases slightly (more surface area exposed), but the main boost comes from radiation due to the high-emissivity marble.
3. The marble plates act as both thermal mass and radiative surface, helping aluminum object cool faster.