I was browsing through the source from TP Link's gpl downloads. It looks like they're already using OpenWrt (the AX6000 source I saw was using a 4.1 Linux kernel)
There's a config file with the following in it:
OpenWrt version: Attitude Adjustment (r2447) (in iplatform.config)
with a log time stamp of Wed Dec 10 10:51:06 2014!!!