Good post, TT. Basically I know what you say is right, I just hadn't seen it stated that the work has been done to rule it out.
And why do I "know what you say is right"? Because I know from a lifetime love of operatic singing, that you cannot tell the nationality of the classically trained singer from the sound of their singing. That's because they are using their lungs, rather than throat/speaking-type-singing.
Compare eg the black American Jessye Norman's versus of Strauss's 4 last songs to that of the white German Elizabeth Schwarzkopf, and that of part Maori Kiwi Kiri Te Kanawa's version - no accent, racial or similar differences at all, and all three with fabulous beautiful voices. (None of the three do RAP though!)
(if anyone is interested, check out the informative comparison of techniques operatic vs pop at https://www.theguardian.com/music/2011/jul/28/can-opera-singers-sing-pop)
