The RK3399 does have a rather limited ISP, not something you'd want to pump 88M in. If you were to use only 1 camera and combine the two ISP nodes it has you'd have a maximum of 26M. This is also why they used a IMX258 as anything better would require to use the second interface for that single camera too.
But to clarify: it's not really as much about "any ISP" as it is about a good ISP that can do multi-camera composition. The ISP in the RockChip SoC can't even do one, it mostly just does basic cropping/rotation/resizing and controls the PHY. While technically an ISP, it's more like an interface driver at this stage. The driver is open source, but the hardware isn't. That's not always a bad thing, as in this case the hardware doesn't run on any RT blob, it's mostly just a bunch of registers for (image) stream processing.
I think in the android rom hacking world, most ISPs are used with their binary blobs (both firmware and kernel modules) but without the configuration and user land blobs, resulting in working cameras (at high pixel counts) but really bad image quality.