Rafa was a slam contender at all the slams from 2008 on (at least SF in every slam that year).
So if the Nadal-Fed h2h is so important, then Nadal should have stopped Federer from winning USO 2008, RG 2009, Wim 2009, Aus 2010, Wim 2012.
Contender? Yes. It doesn't mean it's an automatic win for Nadal though. And even if they met in those majors, the H2H is not 28-0, it's 18-10 for a 64% winning rate against Roger. You mentioned the majors that Roger won from 2008 onwards (even the ones he didn't play Rafa, for some reason) – totalling five – how about mentioning the ones that Nadal won during the same time frame – which totals eight. So, from 2008 onwards, of all the majors won between the two of them, the ratio is 8:5 in favour of Nadal, for a winning rate of 62%. Seems to correlate with the H2H percentage.
Then Fed would be on 12 slams, and Rafa on something like 16. In that case, the h2h would actually matter.
On the contrary, if Rafa led the slam count as well as leading the H2H, the H2H wouldn't matter because it's normally a given. If Person A has more slams than Person B, it stands to reason that Person A must lead the H2H so it doesn't need to be mentioned and won't be as important – see Sampras-Agassi, Borg-Connors, Lendl-McEnroe. What makes the Federer-Nadal H2H strange is Federer has six more majors than Rafa but has eight more losses in their H2H. A difference of two or three wins could be handwaved but an eight win difference is pretty big (and 28 matches is a large sample size for this sport). I'm certain you won't find another player who has led so much in the slam count but fell behind so much in the H2H, which is why it's always brought up. Personally, I attribute it to being a matchup issue, similar to Edberg-Courier, but it is what it is.