Nintendo's site lists HAL as developer of MP6. We know MP6 is going to be different somehow, so maybe it's true. However, they also list HAL as developer of MP5, but I'm pretty certain that was Hudson. Having said that, the earlier games are listed as Hudson, so it's not like they're unaware of their connection to the series.
So there's a mistake somewhere, either with MP5 or MP6, we'll have to wait to see which.