That might be good advice for learners, and a good rule of thumb for all of us. But it's not actually true.
Here's how dictionaries are made. Lexicographers go into the community and find out what words are being used. If the usage of a certain word passes their editorial criteria, it goes into the dictionary.
So, words must exist before they appear in a dictionary.
Also, the correct definition of "the dictionary" is problematic, as we've seen.
I imagine the time lag between a word first coming into usage and then entering the dictionary can sometimes be years. First, it has to come into usage, then it has to be accepted by the community within which it is used, then accepted by a wider community before it comes to the attention of the dictionary's compilers. There is then the process by which the word gets recognised as an acceptable neologism, and even after that, you will have to wait until the next edition of the dictionary before the word gets an entry.
To go back to 'addable': I think my comment that it is rarely used might be misleading in that it suggests it should be avoided. However, many words are rarely used but are no less usable; it is more the case that the context in which they are used is rare. I suggest that this is partly the case with 'addable', which I believe is scarcely used outside the field mathematics, and that is why neither Rover nor I immediately recognised it. But I also maintain that, even where it is used, you'd be more likely to rephrase, eg 'X can be added to Y' rather than 'X and Y are addable'.