Well, perhaps you didn't mean to write it as such but...
I wonder how many other errors have been made because someone was diligently scanning outside in cruise rather than monitoring inside.
...implies that scanning outside precludes the ability to also look inside. Or, for clarity, "rather than" means "And not".
Therefore, your implication is that one
and not the other can be carried out. Further implying that you can't, or won't, do both.
Nothing wrong with my understanding of your sentence as it was written.