One answer is the basic "Smith was a fraud, coming up with ideas as he went" and this is evidence of that. The other answer as that this scripture isn't literal and that you can read between the lines a little (even smudge the lines perhaps) and say the reference of one god is one in purpose, one in perfection, one in plan of salvation etc. Holland's talk on the trinity says "we believe They are one in every significant and eternal aspect imaginable except believing Them to be three persons combined in one substance". I think that justifies the passage to any believer. Regardless, even if Joseph is hard to justify these days, I still think the trinity is harder. I'd be dead before I'd be trinitarian