Ironically I evidently wasn’t clear with what I meant earlier -
I am not disputing that it’s “Niner” - categorically it is.
It’s the pronunciation and annunciation of “Niner”, typically by our American friends that puts enormous over-emphasis on the “-Er” such that it stands out distinctly as a separate syllable that I disparagingly refer to.
My peer trainer in my first role on the railway was a former RAF Air Traffic Controller of 20+ years - he taught/corrected me to say it as
@AM9 put upthread: Like “nine” but with an emphasis on the second “n” sound. Same with “tree” and “fife” they don’t need to be laboured into different sounding words; it’s a more subtle change to clear up ambiguity.