Either both contracted the software out to the same third party company who reused code or one contracted the software out to the other.

I've personally seen and worked on, many times, code originally developed for one customer reused in another project for another customer. Mind you, this is totally legal the way our contracts are worded.

For all we know WD could have paid DLink to produce the whole damn thing and just stick a WD logo on them. That sort of thing is far from unheard of.

Possibly implemented by the same third party vendor.

Looking at the code, it was the cheapest bidder from Upwork.