I think the journalist had an imprecise grasp of language classifications. Indo-European is a "Language Family". It is broken down into 12 "Branches". Of those 12, four are: Celtic, Latin, Hellenic (Greek), and Germanic. The Germanic branch is then broken down into "Languages", one of which is English.
Saying that Indo-European led to Celtic, Latin, Greek, and English is not wrong, but it's sloppy and (I feel) a bit misleading -- since it lumps English in with Language Branches and makes English seem a bit more significant than it is (some the standpoint of linguistic history/genealogy).
It's also incomplete, since, as you point out, there are many other branches and languages that are not mentioned at all. Again, it is not "wrong" to provide an incomplete list of what Indo-European led to, but it is perhaps a bit misleading.
Some of those branches are supposedly more closely related. Linguists once distinguished between the Satem and Centum languages. I don't know if they still do, but the Germanic, Italic, Celtic, Greek (Hellenic) and Tocharian languages appear to fomr one sub family while the Iranian and Balto-Slavic languages form another and the Anatolic languages another still.
Journalist unfortunately possess a superficial grasp of anything they choose to review, be it in the scientific or the political realm. Unfortunately, it easier to point out their shortcomings in the scientific areas.