I don't know that that's correct. That there exists a ligature character doesn't mean the ligature is a character of the language.

It could, mind, I don't know dutch. But in french "œ" (which has a ligatured character as you can see) is canonically equivalent to "oe". It is not a separate letter of the alphabet even though:

* many words should not be written with the ligatured form

* many words should be written with the ligatured form

* it has a different pronunciation than the base form