Are reference designs assumed to be license free?
In the most general way, yes.

The document will normally have some sort of legal disclosure somewhere. Many companies will have a disclosure that you may not use their method unless you are using the part it was designed for.

Not to get into anything too much legal, if you are doing something small, no worries. If you are doing something very large scale or that you expect to make a large amount of money from, just get a lawyer's opinion, in writing. Then if someone goes wrong you are protected.

If it is a method they patented, and they are disclosing it with the intention of you licensing it, which I have never seen done without an obvious disclosure, then you do not want to use the technique.

Please let me know if there is anything else I can add to make this more clear or more helpful. I am not a lawyer of any sort, please double check with one if there is substantial money at stake.