I don't think that's true. I think Leto's Joker will feature heavily in the next Batman solo, even if he isn't the sole villain of the piece. As for whether it will be about Batman trying to understand and get ahead of The Joker, we know that won't really be the case because it won't be their first meeting this time, as has always been the case in other live-action Joker appearances, and I'm glad for it. You can't top the kind of first-time conflict we saw in TDK.
If anything, it should be really unique and interesting to see a Batman who already seemingly understands The Joker, knows what kind of threat he poses right from the start and to not underestimate him, has faced him before, and has a history with him and his antics. How does he approach stopping him and taking him down? What can The Joker do to Batman to hurt him and get under his skin, having already killed Robin and God knows what else? How do they both view each other and act towards each other after all this time? After a decade a more of coming to blows with each other, what will be the outcome of their next conflict? What are their conversations going to be like? So many amazing possibilities, outside of the usual "Who is this man and how do I stop him?"
I really can't wait to see their next meeting unfold.