The NFL guy is right in some aspects for example your are completely correct when saying that outlining the most popular sport in the world can not be described by the amount of countries that play it because maybe these countries only have a few supporters of say soccer. Though i believe your theory for the most popular sport is completely wrong and very ignorant, the greatest thing about soccer is the fact that anyone can play it all you need is a ball this is the reason why soccer is the most popular not because it may be the most interesting for someone who can watch thousands of sports but because for people in the world who have almost nothing like maybe brazil or africa all you need is a ball to entertain you for hours. This is unlike Gridiron as these people cant afford those giant goals a marked out field and a bunch of padding, only needing a ball is what makes soccer so popular. Even though i went a little off topic this explains why you cant outline the most popular sport by attendance or viewers because a large amount of supporters cant afford to go to games or maybe even own a television. I realize collecting statistics for this is probably impossible but in my opinion any other information will never be completely accurate. Anyways say you consider your views to be correct and you would judge popularity by viewership and attendance then if your statistics are correct the NFL is more popular than any single soccer league maybe even the premier league and la liga combined, but and this is a huge but NFL is only one league for a whole sport and if you were saying what is the most popular league in the world you would be correct. I'm 100% sure that the NFL doesn't get as many viewers and people at their games than la liga, premier league, serie a, ligue 1 and bundesliga combined not even considering other leagues like the Russian, Greek Portuguese, Turkish, Swedish, Holland, Korean, Brazilian, Argentine and Japanese leagues. Now you might say this is because NFL hasn't spread to other countries well that is another aspect that shows how NFL isn't the most popular because even soccer is continuing to spread with it only having established leagues in Australia and Qatar for the past decade and the MLS since 1993. Now to your comment on the fact that NFL teams are worth more that is also correct even though Man United is worth more than all of them but consider my point before all the NFL teams combined are not worth as much as all the teams in the leagues I stated previously combined. Lastly the Superbowl is not watched more than the world cup, only because you are American and finish watching the world cup (even when im sure you don't watch it at all) when your team is knocked out it doesn't mean the rest of the world does, if you ever watched a world cup final you would see there are supporters there not just from the two teams playing but all the teams that participated. A world cup is much like a superbowl (yes i watch that too) people go because its a great event and want to experience it not just because their team is playing. That whole statement is not considering the fact that even the Champions league final (if you know what this is) gets more viewers around the world than the superbowl (its true go check). So yes soccer is the most popular sport in the world and whoever that NFL guy is he should really re-think what he said because it isn't right.
Some Ruff Statistics (http://sporteology.com/top-10-popular-sports-world/)
#1 Soccer: 3.3-3.5 billion fans
#7 Table Tennis: 900 million fans
#9 Gridiron: 390-410 million fans
I actually like Gridiron by the way even if they only play for 10 minutes when i watch it for a couple of hours :)