I agree with Thalassamania to a point - it's immediately obvious when somebody starts swimming whether they can do it or not. I often hear of instructors banning kick-rolls during the OW or DM swim tests in a pool because it's "cheating" - sorry, but that's crap - if you can swim well enough to perform a kick roll, and it's good enough for the olympics, I do not care in the slightest if it shaves a couple of seconds off your DM swim tests - I'm already impressed.
Most people can't swim very well. It's slow and technically poor, but they can make the distance without drowning. My main concern is that they are comfortable in water - not because they may have a long swim back to shore after the boat capsizes, but because they are comfortable in water. I find it very difficult to imagine a situation where a diver, in any circumstance, gets into problems and ditches all their gear thinking "I don't need these fins or this mask or these indispensable buoyancy aids, I'm going to get rid of them and swim back to shore unaided"
I find the logic that "a diver needs to be able to swim because they might fall off the dock or boat" a poor excuse - because then we must never allow anybody onto a dock or boat or pier or sea front or beach who is not able to swim. What about all the people who ride river boats or glass bottomed boats or sit on the back of a rubber ring being towed by a speed boat. What about all those folks who ride passenger ferries or take water taxis up the Hudson or go for a romantic gondola ride with a man who has a silly hat and a long pole and a large ice cream in Venice, or a pedalo or rowing boat or canoe or kayak? Surely if we apply that logic then every person who ever gets on a boat, ever, must be able to demonstrate they can swim comfortably in case their vehicle sinks.
Just like the instructor in the OP's post - as soon as I see somebody in the water I *know* if they are comfortable or not. Are they competent swimmers? Well they must be able to meet the certification requirements or I cannot and will not teach them, but if it takes them 30 minutes to swim the 200 metres and they finish up relaxed and refreshed and happy then I'm not going to tell them they can't go diving. Better this than somebody who jumps into the water and exits 20 seconds later like a penguin with a rocket up its backside...!
Me personally - I'm not a great swimmer - I can do it, of course, but as a pastime I hate it. too much time at the surface, when where I really want to be is below it.
Safe diving / swimming all...
C.