Great Question!!
I’m inclined to believe the manufacturers….
Probably were weary of the reported metallurgical failures & proactively sought to improve the product line . IMO .
Hence DLC … if I Was In The Market For Lifters I Would Buy These !
Not Familiar With “ Diamond like coating “
Do your own research & you’ll probably like it too !
PS
I’ve Never Had Any Cam Or Lifter Issues , From , Stock. , Rhodes ……
Of Course I’m A Big Fan Of STP !
Royal Purple , Amalie , Penzoil , Amsoil ….
You get the drift ?, buy the best lubricant you can !
Mopar2ya !
John
IMHFO, the process, It's snakeoil...
but it may be good, not really familiar with it's use,
if it's on a crap/low quality part, doesn't make it better
just outside the realm, stds for me...

& do a proper break-in procedure,
don't let a fresh build or cam change low-idle a bunch before break-in either
15-20 mins high idle 2,000-2,500 RPM at a min.,
get the timing close & deal with it later, carb too
Mandatory use the good break-in oil, w-ph/zinc, full running temps,
& let it cool completely after (
full heat cycles)
many don't/won't, don't think it's necessary

maybe because, they don't know ?
or too cheap
(
or by cheap or foreign discounted parts, for a performance build 
)
or just don't take the proper time
& aren't going thru the full heat cycles, seems people are impatient today
don't drive at a constant RPM, keep it up or down for the 1st 500-1,500 miles
**don't overheat it**, change out the break-in oil,
with a good oil phosphorous-zinc additives
(
it's the engines blood, why would you skimp there ?)
& then the cam starts to & gets wiped almost instantly,
with-in a short time at a min.,
or prematurely, at least
I've never had an Flat Tappet Lifter failures
ever, no matter the brand
because doing the proper break in, & using the proper oil, good oils
maybe ? I'm just lucky, I doubt it thou, luck hasn't been my friend...

Experience & what works has been...
I just now prefer Hydr. roller or solid roller for wilder builds
superior camshafts designs...
I do a similar break-in with them even, it doesn't hurt anything
& could show problems you wouldn't have caught otherwise, can't hurt
"if it works, don't veer from it
"
That's my experience, many others may claim to have different results,
maybe doing it differently, I don't know...
good luck