In addition to Xi, could 国之大者 also refer to a kind of loyal-to-Xi vanguard within the Party? Instead of broadly "all people in his lead," I think it is more limited. If so, the Guancha post above was a description of such a cohort, and perhaps an exhortation to raise up such a cohort of "National Great Ones... who must have confidence, closely follow what the Center (i.e. Xi) cares about and emphasizes," and so on. (对国之大者要心中有数，关注党中央在关心什么、强调什么，深刻领会什么是党和国家最重要的利益、什么是最需要坚定维护的立场)

If this is so, we could expect upcoming profiles of various "National Greats"--some contemporary corollaries to Lei Feng.