The specific quote from sage advice, "Official rulings on how to interpret rules are made here in the Sage Advice Compendium by the game’s lead rules designer, Jeremy Crawford (@JeremyECrawford on Twitter). The public statements of the D&D team, or anyone else at Wizards of the Coast, are not official rulings; they are advice. Jeremy Crawford’s tweets are often a preview of rulings that will appear here."
Whether or not tweets are deemed "official" by wotc, when the same person that makes the official rulings (Jeremy Crawford), tweets about a ruling then by virtue of him being the game's lead rules designer and by virtue of him being the person that puts out the official rulings in Sage Advice, that makes any communication he gives about the rules the closest thing to official that's possible (unless it demonstrably contradicts some past official rule or ruling). In this case about shield master that's not the case and so JC's tweets still remain the closest thing to official rulings we have for topics that aren't clear in the rules or directly discussed in Sage Advice.