This isn't really true.
Because we have seen Igneel vs Acno, and Igneel pushed Acno to a high-extreme diff.
IMO the narrative implies that Igneel was at this full power, for the duration of the battle (that was the entire point of coming to the future). Since Igneel had no soul, soul steal hax couldn't be used on him either.
Ignia is basically being treated as Igneel 2.0, maybe even slightly stronger. And by scaling to Ignia, we can scale the DGs to Igneel as well.
If you restrict soul steal hax, any DG can push Acno to a high-extreme diff. You just need the physical stats to compete with him. Yes, magic don't work on him, but you can use magic attacks to cancel out his magic attacks too (i.e magic can still be effective defensively).
On Topic:
Selene loses low-mid diff, cause her human form doesn't have much physical stats going on for her. But if their dragon forms fight, Selene can definitely push Acno to a high diff.
At this point, Mashima could make happy one shot the next DG and I wouldn't be surprised. The dude has lost all credibility as a writer for me.