Unfortunately that is just an ambiguity in the language here.
“May not” in this case means that you are not allowed to add a transaction to the back stack if you use commitNow(). It does not mean that doing so may or may not work.
If you attempt to do so, you will get an IllegalStateException. See here: https://github.com/android/platform_frameworks_support/blob/432f3317f8a9b8cf98277938ea5df4021e983055/v4/java/android/support/v4/app/BackStackRecord.java#L628-L632