Trump, like any authoritarian nationalist leader, identifies his own interests with those of the country. So what's good (or bad) for him is good (or bad) for America.
Thus he can easily frame any narrative that attacks him - even if, as in this case, it's just quoting his own words back to him with some objectively true data juxtaposed with it - as un-American, or anti-American, and possibly even treasonous.
And this is even more important in a time of war. Of course the USA hasn't been openly at war the whole time he's been POTUS, except for their ongoing low-level engagements in various parts of the world. But he's figuratively at war with the "deep state", he's initiated a trade war with China, came very close to open confrontation with Iran and is now at "war" with a virus (even if he's doing as little as he can to actually fight it, apparently).