It may be a simplistic way of looking at it but:
At 10 minutes before the start, a boat is not approaching the line to start thus a committee boat would be an obstruction this rules of Part C apply.  
However, a one minute to start, boats are approaching the line to start. rules of Part C do not apply.

In this case, -10 sec to start, Blue in starboard tack, is stopped waiting the start top.
I think that "boats are approaching the line to start" so section "C" is off.....so Rule 19 not apply.

Is correct ?

I think 19 does apply between red and yellow because the obstruction is blue, another boat racing, not a starting mark as mentioned in the preamble.

Both Red and Yellow are required to keep clear of Blue. Therefore Blue is an obstruction for both Red and Yellow. 
Blue is not  a starting mark surrounded by navigable water or its anchor line. So, RRS 19 applies to Red and Yellow.

Gordon and Vince:  Yes, that's exactly why the wording was changed. The previous wording implies that rule 19 does not apply, because it didn't specify what obstruction was being considered, and that's not the intent of the preamble. 

Another example is at the pin just at the starting signal, with PL and PW coming up to the mark on port tack and S on starboard tack. Under the pre-2025 preamble, if PL ducks S she does not have to give PW room to go between her and S because PL and PS are both at a starting mark and approaching the line to start. Again, this was not intended. 

The interesting thing to me is that this problem escaped notice for so long!

Under the previous rules, I would have thought that Red and Yellow are not 'at' a starting mark.

Thanks Rob.  I had not fully appreciated why the change was made.  I intended to look at the background for this, but had not done so yet.
-V