Was this a cheat or what ?

I clearly thought these would cost 40 mass, Double the price.

The drain of Nukes sometimes flickers to 80M/s. I don't know why but it is common.

my probably incorrect assumption is that when a nuke is being assisted, whenever the assistance provides enough bp, it temporarily doubles the nuke build speed for a tick to accelerate its progress. there is no actual nuke unit being built so i believe this is how the code spaghetti works

When engineers assist a nuke launcher, the engineers have an energy cost, but no mass cost.

The mass cost for the assistance is in the nuke launcher itself. That's why it goes up from 40 to 60 to 80 etc. depending on how much assistance.

This also means: engineers assisting a nuke can take advantage of the nuke launcher's adjacency to mass fabs. If you ring a nuke with 4 T3 mass fabs, the missiles cost half as much mass. EVEN IF you have 100 engineers assisting it. That's why I like to make 4 fabs + 1 nuke +50 engineers assisting. You get fast nukes at half price.

Edit: Also, the T3 mass fabs have to be completed and I think they have to be running (not power stalled) when you START making the nuclear missile, in order to get the adjacency bonus. After the missile starts making, you continue to get the full adjacency bonus until the missile completes, even if you power stall.

I thought the mass cost showed zero because the build cost was so high that whatever fraction of 1 mass the engineer was spending got rounded down to zero? But I know missile build mechanics are strange, wouldn't be surprised if that assumption was wrong.