Maybe throwing an "of" in there will improve it: "They're not playing that good of baseball right now." Generally "that good" followed by a noun seems to need an "of" like "She's not that good of a secretary." I was also thinking "He didn't do that good of a job," but "He didn't do that good a job" also sounds grammatical to me.
Of course, there is what I think is an unquestionably fine alternative:
They're not playing very good baseball right now.
But the announcer could have intended "that good baseball" as noun itself, i.e. that particular type of baseball, something like:
They're not playing "that good baseball" right now.
or
They're not playing that "good baseball" right now.