OK. So the firs
You said it yourself, the boiling concentrates the flavour compounds, so you will end up with relatively more falvour compounds than the batch that had less water to go with from start.
The way I interpreted your example also had the same mash efficiency. But doing it your way...
So the lower gravity higher preboil volume batch has diluted roasted barley color/flavor as compared to the higher gravity lower preboil volume batch. No disagreement there. Now, to get the same post boil gravity for both batches, the lower gravity higher volume batch with diluted roasted barley color/flavor will have to be boiled longer, thus concentrating the roasted barley color/flavor.
If you believe that your example proves your contention that "The correct way woul be something like "create a wort with an OG of xx and include xyz g of roasted barley in the mash", please flesh out the numbers in the example, show the math, and show the logical connection to your contention. I won't respond to arm waving, and if that's all that ensues, I'll rest my case on my posts thus far.
You said it yourself, the boiling concentrates the flavour compounds, so you will end up with relatively more falvour compounds than the batch that had less water to go with from start.