Minato's question didn't look like a joke. But you could say Naruto's reaction in the anime was. Doesn't matter to me tho, Naruto was adorable there.
The way Naruto thought about it and his really happy answer could be interpreted as a joke. In the manga he seemed way more normal and serious.
But of course, the argument against NS still falls flat. Whether calling her his girlfriend was a joke or not, Naruto doesn't joke about who he likes and he said that when Hinata was in the area. Whether anime or manga, the result's still the same. Saying Naruto doesn't like Sakura anymore and likes Hinata is clearly proven wrong in both versions.
There's no point twisting it as anti.
The thing is people who despise NS sees it joke as in "Just kidding, I love Hinata" or whatever. In other words, this isn't what they were looking for. Though I think that smile really made it worse. It's funny because of that, it has good comedy but at the same time, a serious note. It reminds me when Naruto went to sleep after dealing with Sasuke at Kage Summit Arc. Falling asleep and make Sakura going "what the hell?" only to end on the good note.
How bizarre (not really), but when all considered, anime has Naruto loves Sakura till the day he dies, Sakura continues to wink the audience that she loves him, no longer loves Sasuke as in pointing out without being ambiguous, and Hinata just being Hinata. If you ask me, anime made it too easy.