Server coding should implement wether a primal ancient drop is rare or not such as focus and restraint or compass rose or travelered pledge 1% or 0.01%
Primal drop Chance is estimated to be 1 in (400-500).
I don’t understand your references to F&R or Endless Walk set.
He’s probably saying the rarer items should have a higher chance of rolling Primal… except that those sets he’s mentioning aren’t particularly rare.
Didn’t they condesed the loot pools?, before you had a common, rare and very rare legendary pool pretty sure they left just a common and a rare pool, common being all the trash legendaries that do not even have a power or have a useless one and then all the other ones including sets are in the rare pool.
They changed the weights for rarity, bumping up items like the Star of Azkaranth from its original weight to approximately 3x its old weight, but there does still exist a disparity in drops between many items. This is what is called weighted RNG. Basically, it uses both a base drop chance table and then also a modifier against that drop table to increase rarity. That’s why the Star of Azkaranth and Mara’s Kaleidoscope are still exceptionally rare among amulets for example.
It’s not nearly as bad as it used to be, but you can still have trouble finding certain items in the non-class specific drop tables that also have a weighting applied to them.
“The rarest items are no longer as rare” was patched in, a while back. Tasker and Theo, Wand of Woh, and Kridershot all come to mind. Those items used to be so hard to get. I remember the first time I got a Tasker and Theo for my DH Sentry build, I ran around the house yelling, “Tasker and theeeeoooooo!! 
”! LoL
SOJ clearly isn’t patched the same as unity, and increase in damage allows for higher off sets r u saying they are legendary aren’t on the same tier as sets