Hume pointed out that no number of propositions that use the copula "is" can ever logically entail a proposition with the copula "ought". Yet, he says, we often see philosophers slip into "ought" conclusions without ever explaining how they did it. That's a shrewd observation, and we should not throw it out in the name of being able to study morality more easily. It imposes a discipline on us, that if we start introducing "oughts" we must explain how we did it, and it can't simply be a logical entailment from a string of "is" statements.
But, as I noted before, the Statement interpretation is not at all tenable. Blackford is right that Hume said that authors of systems of morality shift from propositions with 'is' and 'is not' copulae to propositions with 'ought' and 'ought not' copulae without explaining their shift or even recognizing that it had happened. But it is trivially easy to find propositions with the copula 'is' that logically entail propositions with the copula 'ought' (if we assume for the sake of argument that 'ought' counts as a copula); for instance, "We ought to do good" is a true proposition entails We ought to do good. Disquotation is actually not even necessary, but it shows immediately the problem with putting Hume's point in terms of logical entailment. Hume's reason for his observation was not this but that moral rationalists of his time regarded obligations as necessary relations perceived by reason alone and Hume was arguing throughout the relevant section obligation cannot be a relation and cannot be perceived by reason alone.
