I think what people really mean when they say "This can't be tested" is:
"The cost of writing these tests outweighs the benefit", which often is a valid argument, especially if you have to do major refactors that make the system overall more difficult to understand.
I do not agree with test zealots that argue that a more testable system is always also easier to understand, my experience has been the opposite.
Of course there are cases where this is still worth the trade-off, but it requires careful consideration.
"The cost of writing these tests outweighs the benefit", which often is a valid argument, especially if you have to do major refactors that make the system overall more difficult to understand.
I do not agree with test zealots that argue that a more testable system is always also easier to understand, my experience has been the opposite.
Of course there are cases where this is still worth the trade-off, but it requires careful consideration.