It depends on what you mean with "practising engineer", I assure you that design engineers at Intel, AD, TI, RF companies and so on use much more than that. If we are only speaking about audio and DIY, perhaps it would be true what you say, still, I dont think it is enough.It's not the Gospel. It's a PRACTICAL book. The theory and eqns it has are those used by a practising engineer.
However, most people might be perfectly ok with the "Transistor Man", I prefer more to know how it actually works.
P.S. I mentioned I liked its practicality, and that I think everyone should read it, but, some people think its all you need, and I disagree with that, there is also some sort of cult following and praise behind it that I don't think is justified.
Also, are you a guru in your old age?