It’s not true.Actually as far as watching on TV the NFL is the second most watched sport in Germany. Not to play but to watch. You can find a million articles online about it.
Formula One has an average TV viewership of about 2.5 million on RTL (a german free to air TV channel), while NFL has about 0.7 million average viewership on RTL.
Then we got Handball, which has an average TV viewership of about 5 million at World Cups and European Championships (which take place every year). You will have much more people in Germany who have watched a Formula One race or a Handball game in their life (or in the last year) in TV than people who have watched an NFL game in TV. Much, much more.
The NFL takes place from September until February and has a certain relevance during that time as a TV sport in Germany (but, like I said, much less viewership than Formula One and Handball, and comparable to Tennis Grand Slams), but American Football nearly has no relevance outside of those NFL games in Germany.
Meanwhile, 1.5 million people in Germany are registered Tennis players, 800k Handball players, 250k Basketball players, plus these sports like Handball, Basketball and Ice Hockey have domestic leagues who have a certain relevance in the country.
So no, overall, American Football doesn’t belong in this list of the most popular sports in Germany.