Bitcoin is a productive asset. I buy it, lend it to a business who can use it as collateral and I get paid back more bitcoin than I lent them, as compensation (i.e. interest)
That is perhaps stretching of what is usually meant by a productive assets (you can make the same argument for a piece of art, for example, or really anything). Typically, the asset itself is productive, not that it can be used in a productive activity (e.g., gold as an non-productive asset even though you can lend it out).
