The agent dealing with your case may have a different take.

On top of this "flat fee" to revive the ticket, they may also decide to charge the fare difference for the journey now actually being flown. Particularly when the "missed" sector is the very first sector, and the passenger made no attempt to contact the airline in advance and gave every appearance as only ever intending to depart from CDG rather than from BCN, I think you will run into issues over and above this flat fee.

Note that the text on the e-tickets refers only to a "flat-fee" for "non-compliant use" - so payment is, in effect, admitting the non-compliant use. (If you had a genuine sob-story to explain why you missed the segment, this fee wouldn't be charged). It's being a bit foolhardy to then think that, having basically admitted "non-compliant use", that that fee alone is the only negative consequence; in more cases than not, I would expect a fare difference to be levied, too.

There's certainly nothing in the text that prevents them from also doing a fare recalculation; and carrying over the value of the ticket towards the new fare. Just because this topic isn't explicitly pointed out in this part of the text we are all fixating on, does not mean that this is not how they would proceed.

We can argue this point this way and that, and a lot may depend upon the agent in question; however, I really do not think it is an advisable strategy to consider that buying ex-BCN tickets and only ever commencing your travel at CDG will consistently lead to this fee being considered a perfectly "legal" way to depart from CDG while using the prices set for the BCN market.