An OEM firmware being "based on" OpenWrt (something very common among most vendors) is not the same as actually running real OpenWrt.
If a commercial firmware is "based on" OpenWrt, this typically means the SOC manufacturers takes a random version of OpenWrt they deem to be convenient as a base (this doesn't necessarily correspond to the source of an official release, but may be a random unmarked snapshot), they then typically rip out OpenWrt's kernel and replace it with their (usually older-) semi-proprietary kernel, with plenty of unofficial/ unmerged or closed source drivers/ modules or other changes. If you're very 'lucky', they even switch to non-standard toolchains instead of gcc+binutils+musl, etc. Once the SOC vendor is done with it, they pass this SDK to their customers, the router manufacturers, who typically rip out the webinterface and other user facing components, often including the config store (e.g. uci --> nvram) to replace them with their own 'easy' replacements. What you as a customer get access to bears barely any resemblance to "OpenWrt", but it has obviously saved the vendor tons of time during the development. The result of this is that there's typically no compatibility with real OpenWrt left, it needs different ways of configuration (because they've at least partially replaced uci with their own system), it's not compatible to OpenWrt's package repos (different runtime library ABIs, different configuration means, etc.), you simply can't do quite a few of things you could do on normal OpenWrt. As soon as you're looking at anything remotely touching the kernel (security fixes, new features (drivers), etc.), you're typically out of luck (as at least parts of the crucial drivers, e.g. for WLAN, tend to be missing). Just as an example, the current wrt32xx OEM firmware identifies its kernel as "4.4.14", while the current 4.4 LTS kernel would be "4.4.181" at this moment - do you really think that Marvell has backported all relevant security issues from 4.4.181 to 4.4.14 (without updating the version number) - and then made a new SDK available to Linksys, who has then rebased their changes on the hypothetical SDK update?
Depending on your requirements, you may be willing to accept those tradeoffs in 'favour' of better(?) WLAN support via their proprietary drivers - but "based on" OpenWrt just has very little in common with real OpenWrt.
 and you may even find a third step inbetween SOC- and router vendors, the ODMs designing parts- or the whole router and its firmware for the "manufacturer" (respectively trademark holder) on the label.