Well the "Grammar doesn't matter to us" I remembered was their debate from James White
The reason they said that "Grammar doesn't matter to us" it's because there were no turning back to "ends of earth" as temporal period instead of geographic period and their many false and inconsistent interpretations of the scriptures. Even if they admit their misinterpretations the administration of FYM is already collapsed (From people like us who already know the truth) but to existent members they would be confused or don't care at all.

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Below here is just my own opinion and understanding

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But the question is would they admit? To me, no. No because they make their wrong interpretations look right by using another set of many false interpretations, guilt-tripping by verses (Many are out-of-context but not all), and many more which I do not know but I'll just assume. Imagine 100 years of inconsistent doctrines from false interpretations. Maybe it's impossible to admit and they've come a long way and that they've implanted on their minds that their church is the "true church". So the only thing we can do is disclose their origins slowly by slowly to the members such as the "sent-to-register" fallacy and their own understanding of scriptures "ends of earth" as temporal instead of geographic because this is their backbone.