Earlier quoted context omitted.
This is actually Google's fault. Because Google is large enough Netflix could be nice and have special handling for Gmail addresses but this would be going against the email RFC that says $inbox1+$subaddr@domain == $inbox2+$subaddr@domain iff $inbox1 and $inbox2 are identical . Netflix has zero idea how any particular email operator mangles or maps inboxes to actual users. Apple's private email service maps all sorts…
This is Netflix's fault for not verifying email addresses before taking payment details for a recurring subscription.
But in addition Netflix is right to allow the creation of two accounts where the difference is only dots.