What else do give linguist a credibility:
~Most linguists publish their specialized pair language(s) with target language which is not of their native language. This is understandable for those who have dual mother tongues either by spending an equal amount of time during their lifetime in two countries with different spoken languages, or they've come from mixed-parent (race/nationality/etc.). IMO, unless they have the above circumstances, do not take their claim seriously.

~ to be continued...