Slang history. In the US BFE is well established as Bum-Fuck-Egypt to mean in the middle of nowhere. Explicitly stating Butt-Fuck-Egypt leans towards sexual imagery and is not part of the slang. I completely understand the confusion as bum is commonly used in the UK to mean butt. Someone on the linked site even got it wrong.
As an example of communication differences, in the US it is not uncommon for someone to say "Pardon me" when they are getting the way whereas people in England might assume they farted. Or another example where communications go wrong, someone in the US or the UK might give a thumbs-up when they agree with results or an idea but in some parts of Africa that is a death threat.
For what it's worth, growing up in Texas in the 90s, it was absolutely "butt-fucking Egypt" or "butt-fuck nowhere". I don't think I'd ever heard anyone use the "bum" variant.
As an example of communication differences, in the US it is not uncommon for someone to say "Pardon me" when they are getting the way whereas people in England might assume they farted. Or another example where communications go wrong, someone in the US or the UK might give a thumbs-up when they agree with results or an idea but in some parts of Africa that is a death threat.