Hello everyone,
Speaking about accents and monolingual native English speakers, think about what is really (or should be if the test is actually compliant) tested: the ability to communicate efficiently and therefore safely. Having a high proficiency level of the English language is no good if you use it colloquially on the radio. A high level should be awarded to those who consciously use unambiguous and concise language. For a native or expert speaker this is important that they use language leaving out jargon and slang and omit complex structures.
It is about language awareness
(look here for a native speaker who is really linguistically aware). ICAO just now put an amendment to Annex 10 stating that "Go Ahead" could be mistaken for an authorization to proceed with taxiing therefore should be omitted when making "call and answer" transmissions. The CAA of England has been doing it for decades in all situations as they understood the danger of "Go Ahead".
"The Test" (for those who asked, there are about a handful and of those only a couple are actually compliant) should measure one's ability to use plain language only when standard R/T does not suffice and then they choose to use plain language in an unambiguous manner-structured correctly and efficiently.
Best to all,
PE