To be fair to Paul, he does make the distinction between large scale work on hard woods (gates, big doors) and delicate work on mild timber (furniture making).
The 1970s Marples chisels he mentions, like their contemporary Stanley equivalent, are a lot thicker and made of more robust steel than pre-war bevel edge chisels which taper down to nearly zero along their sides, so it's no surprise to see it demonstrated that they are robust enough for the job.
So it's good information for someone starting out or just wanting the bare minimum of tools.
But for me, now that I have pretty well covered the range of light, medium and heavy chisels for all ordinary sizes bar timber framing and shipbuilding, I will continue to use a mortice chisel whenever I can.
If the mortice is too short to accommodate a deep mortice chisel, I would use a sash mortice chisel, a registered chisel, or possibly a firmer.
On a side note, I'm sure I remember at school that we were set the task of cutting narrow mortices in softwood without using a mallet. I think the chisel would have been an ordinary Sheffield made firmer chisel with a wooden handle, kept properly sharp by our teacher. I think it was a genuine exercise in understanding how the chisel penetrates and cuts the fibres into free space and wasn't just an excuse to keep the noise down!
