Used to, no? I believe Asus has stripped out most of that code by now.
Structurally - it hasn't changed much - the Belkin GPL drop for the MediaTek based RT3200 is an evolution of the classic Linksys WRT platform that Tomato forked from the WRT54G, and Asus forked from there.
From a SW architect perspective, it's interesting to see - esp as this code base has been ported on to about every WiFi chipset ever...
Going back to the QC/MTK SDK's - yep, they're both forks based on LEDE/OpenWRT, and they keep things perhaps on an older kernel for maint purposes as they have to support their respective product lines with something that could be rolled into production - they stay away from the bleeding edge kernels that OpenWRT has, but for good reason - they want to ensure that all their middleware drivers and accelerators are stable...
It's kind of nice, as if one is comfortable with QSDK, migrating to the Mediatek SDK is pretty easy as all the upper layer stuff is the same, and the SW workflows are basically the same - with a little effort, one can merge both into build system that supports both vendors.
I've seen that both Qualcomm and Mediatek rebased their SDK's, usually on a technology jump, e.g. moving from WiFi5 to WiFi6, or CPU arch revision, going from MIPS to ARMv7 or ARMv8, and sometimes both...
And just like AsusWRT, we see questions all the time about - "why the old kernel" - well, because - mostly for reliability and stability for the entire platform that the SDK has to support.
fun stuff