首楞嚴義疏注經 9.105

9.105

滅色取空，非真空性，妄謂為盡，故云不盡，以不能灰滅心智故。言此等者，總指四天也。

They extinguish form to grasp emptiness, which is not the true nature of emptiness; falsely claiming it to be exhausted, it is therefore said that they 'do not exhaust' it, because they cannot turn their minds and wisdom to ashes and extinguish them. The words 'these persons' generally indicate the four heavens.