OK, so one, from 1641 the legal category of slave existed in Massachusetts. Two, even so, they might have been free.
So looking at this with the question of "so did he treat them as property?" I see that he says "yea though they should be disposed of to any other place before by my executors, & if they should be still kept or employed at my ffarme or in the service of my son or wife..."
So he's treating them as property, because executors dispose of property. Richard's daughter Zipora *might* be free, since he doesn't list an owner for her.
A quick google turns up a 1999 article about Angola -- apparently there's a TON of documents about him and his family. You can sign up to read it for free, and yes, they were slaves, and were valued as part of the estate by the executors.
And there's a bit more about Grace and Zipora (evidently the line about Zipora was added later. possibly because Richard died -- or was sold or ran away, I suppose.  And she was valued as property, though the article opines that as a child, at the time she ought to have been considered free.)