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Someone's going to have a better answer than I. But I think the AU is more likely to help the mortgage scores than it would be likely to help FICO8. Try not to report a balance on this card, though. It just gets in the way of optimizing scoring.
More frequently, the concern is that mortgage lenders don't like AUs to begin with. Some feel that AUs muddle up their ability to determine one's "creditworthiness." But obviously, that's not the case with your lender.
Yes, AUs count for the "mortgage scores". Lenders also have no issue with AU accounts between spouses for mortgages - it's AUs from random other people that can trigger requests to remove them.
In general, even for FICO 8, spousal AUs still count - the anti-tradeline-selling algorithms (in theory) count spousal lines to allow SAHMs and the like to still qualify for credit in their own names. But your reports won't directly tell you if that's working correctly...