I am pretty sure it is not aerogel. The material has to be transparent for neutrons, lightweight, and immediately vaporizable to create radiation and/or ablation pressure. Styrofoam fits the bill; usual aerogels do not.
From what I know about the Stockpile Stewardship program, they had difficulties in recreating the exact composition (including impurities) of many weapon components, probably including the interstage — because the only way to make sure that new weapons and their replacement parts will work without tests (which are now banned) is to recreate everything to the smallest detail. So the problem is not manufacturing some exotic material, but recreating the exact composition and impurities.
This is a good point- imagine having to recreate, exactly, plastic foam manufacturing processes from decades earlier. How much shop tradition- "oh, yeah, the official design says we do this, but Bob over there figured out a clever trick that saves a lot of time and now we all do it instead"- might you have to track down or rediscover?
From what I know about the Stockpile Stewardship program, they had difficulties in recreating the exact composition (including impurities) of many weapon components, probably including the interstage — because the only way to make sure that new weapons and their replacement parts will work without tests (which are now banned) is to recreate everything to the smallest detail. So the problem is not manufacturing some exotic material, but recreating the exact composition and impurities.