Being mistaken doesn’t make it not Da’as Torah. It just means that he was mistaken as to what the Da’as Torah was. Even if he wasn’t demonstrated to have been mistaken, it was still just his opinion (albeit perhaps a weighty opinion) as to what the Da’as Torah was. So being mistaken is irrelevant. It is just as much Da’as Torah after we know of the mistake as it was before we knew of the mistake, that is to say it was his opinion of what the Daas Torah was which has been shown to have been mistaken.
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