Because, well… I do!
I do not think he in any way meant the RAM usage would ever exceed 4.7GB of RAM, because that statement makes no sense whatsoever. RAM usage can and will fluctuate based on a whole host of different factors, including (but not necessarily limited to) what aircraft you are in, where on the planet you are, whether or not you have dense or sparse scenery around you, whether or not your plane is moving, what view you may be using and a whole lot more.
I attempted an experiment a few days ago where I simply watched how much RAM the system used from right when MSFS was started, all the way to spawning on the ramp at Charles de Gaulle in an A32NX. I did not take note of which livery, but I suppose that, too, could use more or less RAM depending. It used 17GB.
Windows can will and does stuff that 17GB into some smaller amount for people who simply do not have 17GB to give to the sim, which obviously includes all people with only 16GB total, and could even include some with 32GB. However, I will note that during my test at no point did the total go above (or even close to) 32GB total in use, so I suspect 32GB is the “sweet spot”. I think windows using it’s magic to stuff that 17GB into something smaller simply MUST come with a downside somewhere, and it may be manifesting itself in the form of some of the graphical anomalies others report that me and my too much RAM do not have.
While I am not Seb, and I am incapable of reading his mind, there is simply no way he meant that under no circumstances would the sim ever use more than 4.7GB of RAM. During the scene they showed, perhaps, but it’s too big of a variable to put a hard limit like that on.