Now we see through a glass, darkly...
- Apr 5, 2016
I posted yesterday a burst of 21 shots of a dragonfly in flight using ES on the R5 + 100-500 and according to the EXIF data 20 of those were in one second (frames 2-21 all dated 13 June 2021 at 12:53:33, frame 1 12:53:32) so I don't think Canon is lying.
With the right EXIF viewer you can actually see the sub-second that each image was exposed. I imagine this is the time when the shutter starts to open, or maybe when it closes? With E-shutter that would be the beginning or ending of readout? Maybe when ADC begins/ends? Whatever it is, Canon includes the sub-second in a separate "maker note" field apart from the standard date/time field that does not allow for anything shorter than one second. But most applications that read EXIF info do not display it.