Bullying is a misuse of power.
If you are unpleasant and 'demanding' to someone who can't answer you back in the same way without negative consequences for their job, that goes beyond being 'demanding' and is 'bullying'.
It doesn't matter whether they pick on one or two people, or do it to everyone working for them.
In the example given above of 'leaving so early?' at 7pm - if the junior staff leaving at 7pm couldn't turn round and tell their 'demanding' boss - without negative consequences - that they had every right to leave at the end of their contracted hours, and would prefer not to deal with snide remarks about it, well, that is bullying.
The days when being 'the top in your field' gave you an excuse to belittle others are over, thank goodness, and the damaging and inequitable results of a long hours culture (which tends to exclude women, for example, as they still tend to be the ones to be the primary carers) are more and more widely known.