I can agree the gauges are pretty good, but over how long is the average fuel economy figures calculated? I guess you did say 'real-time' so you are right. I know on my mere falcon, there are separate figures for average economy and figures for instant economy. The instant figures cycle up or down, though to be fair not that fast if you are on a steady stretch of road.
Fuel injectors are a pretty well know metering method. When scaling injectors to suit ECUs there seem to be two different ways of doing it. From memory, one talks about flow during the first instant of time, and then the linear flow rates after that. I can't remember what the second is, but its accurate, just in a different way. This data provided by the fuel injector makers, or sometimes 3rd parties, is fed back into the ECU to give you precise metering, so it makes sense that the fuel economy gauges are accurate.
A comment in that article says "Car computers only know how long the injectors are open vs closed and assume a consistent rate of flow for the open time and thereby calculate how much fuel SHOULD have gone through the injectors in that time.." which is not quite true. There is no assumption for consistent rates of flow, but has at least two slopes to cater for the opening flows and then after that.
They also trim the injector opening times after that, but they are still using these slopes to work out how much actually was injected.