Is powerfulest a valid superlative?

Whenever I type powerfulest in a word document or on here it’s always flagged as a grammar error.

I immediately consulted google and found some entries in low end “dictionaries”, but came across this reddit post:

"Powerfulest" isn't a word, and words are not real words until Merriam-Webster or OED recognizes them.

So, in your opinion what’s the verdict on this one?

Solution 1:

Powerfulest is definitely a less common form of superlative compared to the more commonly used “most powerful” ,but, as shown in Google Books the term is used.

From: The Portable Walt Whitman By Walt Whitman

Over those politicians and great and little rings, and over all their insolence and wiles, and over the powerfulest parties, looms a power, too sluggish maybe,

From Stalker's Creek by Will Cade

He ' s the powerfulest man in town , and the closest thing to a rich one . By now I reckon he really is rich . Hey , want me to ask him ...