How do Chomsky and Pinker's monolingualism explain their flaws about language acquisition?

Monolingualism is a common criticism of many Chomskean linguists. The issue is not so much the fact that the individuals don't speak other languages. Chomsky studied and knows many languages, taught Hebrew and even wrote his early thesis about Hebrew. Also many researchers in the generative paradigms are native speakers of other languages or study other languages.

The issue is that many of the concepts of generative and by extension universal grammar appear as if English was somehow the default. It starts with the first formula every one learns from generative grammar:

S -> NP VP

Why not VP NP or just VP? All of those could be defaults if you started with another language or did not choose to think of a simple declarative sentence as the core of language.

Also, in this scheme, a sentence like 'They like Joan.' may appear as somehow more basic than 'Joan likes them.' which seemingly introduces morphology while starting with other languages morphology would be the default. When applied to language acquisition this results in undue focus on the learning of syntax which in some languages pales in comparison to the complexity of the morphology that has to be acquired - much of with complex and subtle semantics combining both syntactic and pragmatic meanings (think evidentiality or classification systems).

This is by no means just a problem with Chomskean linguistics. I've criticised some concepts in construction grammar and functionalist grammar for the same reasons.

Another issue is that Chomsky (as do most linguists) assumes monolingualism is the natural and default state of language speakers. Whereas about half the world's population is multilingual (and probably even more were so historically).

But this is not by any means the fundamental flaw of Chomsky's (and by extension Pinker's) approach. Just something critics occasionally trot out to bolster their argument. There many much more intrinsic issues that have been covered extensively elsewhere.

Postscript: There is no virtue or rigor to the statement "Learning a new language can be a transformational experience. Monolinguals are in a state of non-transformation in this respect." It's just an empty cliche which in this case can be even more confusing because it uses the word transformation (perhaps in subtle reference to transformational grammar which of course has nothing to do with this at all). People put about a lot of platitudes about language learning and they are rarely helpful and never in any way measurable or easy to capture in a systematic manner.