As a rough rule of thumb, those ratios are approximately right but there is quite a bit of individual variation and there will be more individual variation in the mean maximal power to FTP ratio at shorter durations than at longer ones.
If we consider a mean maximal power curve, a plot of the maximal power one is capable of sustaining plotted against duration, it is generally pretty flat from 8 minutes out to many hours. It's not of course totally flat and there are various individual factors that influence the shape of the curve. The slope is greater at the shorter duration, and it has more individual variability there.
This example curve taken from Jack Mott's website shows (note time is on a log axis):
Now while this is the general shape of a MMP curve, there is individual variability and I chose this image as it gives some clues as to why, namely the contribution of the various metabolic energy systems that supply the energy varies depending upon the duration of a maximal effort, and also upon the unique physiological characteristics of the individual.
So while long duration efforts are dominantly fuelled by aerobic metabolism, during shorter maximal efforts there is a greater contribution to the energy demand by anaerobic metabolism, and anaerobic capacity is highly individually variable.
As a result, the ratio of short duration power capability to FTP can vary quite a bit between individuals, while the ratio of long duration power capability to FTP will be less variable.