Yes, that's true. I guess Sammy's almost the one manufacturer that has complete specs of its SOC (the Exynos ones at least) so they can write the drivers and blobs themselves...
And do not forget Apple The same goes with it.

All others are in the mercy of the SoC vendors.

For some really obscure reason that quite escapes me, HW vendors think that revealing too much in the driver is going to enable other companies to copy their IP and make competing products. (If that'd be really the case I'd be really surprised... competitors already know almost all of their rival's products)

I rather think they are just embarassed of the poor quality of their code, or else they have something fishy hidden there