The time in the EXIF metadata is just based on the time/date stamp that's been set in the camera. I don't know about the newer Canon but I'm pretty sure the older model would just be something someone manually set.
Re-reading the article, I think saying the pics were shot at the "same millisecond" was just a colloquial expression to mean the look of the waves makes it look as if they were shot at the exact same time. They shared metadata but that seems to have been to share the other shooting parameters. I believe (and this would seem to confirm [1]) that EXIF timestamp data is only precise down to 1 second. added: and only as accurate as the photographer who set the date and time.
Both cameras (5DM4 and 60D) can be set via GPS using a Canon accessory, but I highly doubt that is the case (very few own that from what I have seen). Even then, the camera doesn't preserve millisecond data in the EXIF data.
Re-reading the article, I think saying the pics were shot at the "same millisecond" was just a colloquial expression to mean the look of the waves makes it look as if they were shot at the exact same time. They shared metadata but that seems to have been to share the other shooting parameters. I believe (and this would seem to confirm [1]) that EXIF timestamp data is only precise down to 1 second. added: and only as accurate as the photographer who set the date and time.
[1] https://www.media.mit.edu/pia/Research/deepview/exif.html