What's not true?
And estragon, I don't think of the IQP as a weakness, but in the tarrasch where white already has a development lead (with the first move) and gets the ideal setup to play against the isolated pawn white should definately have an advantage.
Back to the original diagrams : there are a few factors that make the Tarrasch position better than the Nimzo for White :
1- the g2 bishop pressure the d5 isolani
2-the Bc1 is free to move along the c1-h6 diagonal, particularly to g5 where it attacks a defender of d5.
3- the Nf3 can not be pinned by ...Bg4
Estragon
Just to clarrify what I was saying: The IQP is certainly a "structural weakness" meaning that it is a weakness in the pawn structure. But as you have quite correctly said, playing with an IQP can be good - and ones position can still be better even with an IQP. However in that case it isnt the IQP which is giving your position the strength - it is the open lines, the control of the square in front of the pawn, the possibility of pushing the pawn, the active pieces, the outpost squares supported by the IQP and the space advantage that give the position strength - not the actual IQP itself. The pawn itself is still a structural weakness - but chess is not all about pawn structure - there are many other factors and of course (as you said) the presence of an isolated pawn certainly doesnt make a position bad - one can have a very very good position with an IQP.
Personally I love playing with an IQP - and I also love playing against one. You also sound like you know how to play IQP positions well.
Just a general note here: If you play well against an IQP, its quite easy to learn how to play well with one. And by learning how to play well with one, you will find that you get even better at playing well against one too. Its a case of looking at the position from the other side of the fence. Set up a common IQP position 10 moves in from a Nimzo/Panov Caro Kann/c3 Sicilian/QGA etc etc etc and play a 20 game blitz session against a friend of about your own strength alternating between black and white. You will be amazed how much better you get at playing one side if you train for both sides of it.
It is not true