Those numbers were made up to illustrate a point. I have no idea how buoyant helium is at STP and didn't intend to conform to any realistic scenario. "7 cubic meters at STP" is a perfectly fine empirical answer, and I guess you can get the mass of 7 cubic meters of helium from the ideal gas law.

The point is, it makes no sense to say "the weight of the helium must be equal to the weight of the helium+cylinder system minus the known weight of the cylinder, just as the weight of a chicken on a plate is equal to the weight of the chicken+plate system minus the weight of the plate". You've got to bring in some extra information.