Back in the bad old days, one could structure one's flying to provide an essentially fulltime availability for other work, including training - might have affected the sleep a little but it certainly was possible.
I won't doubt for a second that what you said is true, buthe thing is, the author wants to be a cadet for an airline. Unless the cadet has flexibility in their work arrangements, then I can't see why that couldn't happen today. Most cadet programs are now full-time commitments and because there are no part-time MBBS degrees available, it just isn't possible.
The easier approach is to do the training fulltime prior to running with the flying job. Such was my strategy and, indeed, I just missed out on completing the BSc/BE double in parallel with engineering (without doing the extra year) as well as completing the bulk of my flying training on weekends. Mind you, back in those days, we averaged one or two nights a week working straight through without any sleep .. just par for the course, as it were. The Arts folk had a more relaxing life but that's the way things were and, I suppose, engineering is not all that much different these days ?
I'm not sure how different it is today but combined engineering students do on average 22-28 hours a week and their degree goes for 5.5 years.
Again, in the bad old days, more than a few airline pilots opted for professional flying on a recreational basis.
Now, if a medico can tie work and recreational flying, what is the subtle difference between the two which might preclude professional flying ?
Again, the author wants to become a cadet. I think our misinterpretation is coming down to our definition of professionally - indeed, getting paid to fly would be defined as 'professionally' (instructing, sight-seeing etc), but I was referring more to pilots in airlines, given that this is what the author is alluding to.
For those who do recreational flying, what I meant by that was those who maybe go down to their local airfield and just fly around for their own amusement.
Main point is that some have combined medicine and professional flying with some success so the option remains valid.
Indeed, if again by professional you mean by those who get paid, then yes. There is no reason why the option isn't valid. But again, this is out of context with what the author is seeking. I think you would be VERY hard pressed to find a pilot in an airline with a MBBS, who practices as a doctor when he/she isn't flying.
Having a back up degree is important because you never know what could happen to your health. By having that back up, it's pretty much an insurance policy. But be mindful of what you pick as your back-up.
Anyway, I think this has been taken a bit too far off-topic now. However...
The important consideration is to do a course in which one has significant interest and can contemplate a career if the flying turns to custard for whatever reason.