[lmilcin]

Stoichiometric mixture is only loosely connected to flamability.

You can have a stoichiometric mixture that will not be flammable because both reagents would be in right proportions but very diluted in the inert gas.

And a mixture that is very non-stoichiometric can still be very flammable, no problem. The extra gas most of the time performs the same role as inert gas -- dilutes the reagents and takes away energy from the reaction. There are exceptions, though, where you can have different results depending on the mix (for example your burning producing CO rather than CO2 because of abundance of carbon relative to oxygen in the mix).