[scoot]

With a few noted exceptions (often where the -ly form has another meaning) flat adverbs are considered archaic at best. That the examples you cite are from early modern English is hardly surprising - American English usage did not develop in complete isolation from its roots.

That occasional use of this earlier form has persisted in American English dialect is also unsurprising; there are other examples of anachronistic language forms, often in slang, topolect or sociolect, where common usage has otherwise died out.

The GP poster correctly observes that non-standard use of flat adverbs is more common in American English, sometimes associated with a "deep South" topolect.

Yes flat adverbs still exist in common and accepted usage, usually for good reason, but this does not make using just any adverb in plain form "correct", or less jarring to the educated ear, unless done so for stylistic reasons (which was not the case here).

I still feel the correction was unwarranted in this particular social context, but perhaps that's just me.