If they had written but they are never to be the only source instead of but they must never be the only source I would wholeheartedly agree with you. The best would be but they cannot be the only source.
However they used the modal auxiliary verb must before never which changes the connotation of the word never from absolute to implied.
Definition of auxiliary verb
: a verb (such as have, be, may, do, shall, will, can, or must) that is used with another verb to show the verb's tense, to form a question, etc.
There is a further set of auxiliary verbs known as modal verbs or modal auxiliary verbs. These combine with other verbs to express necessity, possibility, intention, or ability. The modal auxiliary verbs are must, shall, will, should, would, ought (to), can, could, may, and might. For example:
You must act promptly.
Can you speak Spanish?
I would go if I could afford it.
He said he might reconsider his decision.
I ought to visit my family.
We should get to London before midday.
May I come in?
And as pointed out that must is akin to should in this tense, it reads as should never which is not an absolute that the use of these as a source is forbidden. In short, the sentence is asking us not to do so and not actually declaring the use is totally forbidden.
Language is a powerful tool and as all tools, how it is used is extremely important. This is why you will not see the term must used in legal documents where an absolute is involved. If you ever have to sign a contract where the other party must do something, fire your attorney.