That's one approach or technique, a slow one.
My recollection is Derek saws the waste before the final chopping of the waste. My recollector could be on the blink.
My favorite chisels for paring away the waste of dovetail and other joinery are my thin blade Buck Brothers chisels. With an approximately 15 - 20º bevel the edge is prone to quickly fading or micro chipping. They can be quickly touched up on oilstones and be back to work in no time.
For mallet work there are plenty of other chisels available.
Different folks have different motivations. Some want a minimalist accumulation around their shop. The more stuff there is the more storage space it will take. One can only envy how much easier it is to grab a chisel for paring which is the same chisel used for every other chisel task. Try to imagine the situation in my shop when a decision has to be made about which 1/4" chisel to dig up to be used for a specific task. Then imagine the extra work of keeping them all sharp at their different bevel angles.
If there is one set of chisels that can "do it all," then that is the super-secret chisel technique.
jtk
"A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
- Sir Winston Churchill (1874-1965)