Well hopefully there is no combustion inside of the manifold and having said that, typically the fuel is combusted when it is under higher than atmospheric pressure. And engines work better when the fuel bypasses an intake manifold altogether and is atomized, by an injector, at the optimum location and time inside the cylinder. BUT YES you need a vacuum inside the manifold to draw fuel into the heads and cylinders from a throttle body or carburetor. IIRC sometimes up to 15~30 lbs of vacuum in the manifold at idle, considerably less during full throttle with the butterflies fully open.
And I am not trying to argue with you at all here but there are a lot other considerations to worry about that can make your DC combust aside from static spark.
I cant tell you how many times I have cut through a finishing nail with my TS. Surely there was a really hot spark that traveled into the collector hose.
AND with marginal powered table saws or those with dull blades it is not at all uncommon, at times, for the wood to actually get hot enough to smolder and for the dust from that to go into the DC.
I totally believe the static spark thing is a threat with any DC if the DC is used to clean up something other than wood saw dust. I think the caution labels are a blanket statement for what ever the DC might be used for. Maybe some one uses them to clean up grain elevators. ;~) I know I use mine to suck up anything that is on my shop floor including my son's hair when my wife cuts his hair. Yeah is is 28 but sometimes this is the only way we get to see him. LOL