Most people don't forget that, in fairness. The point people are making is that this applies to Higuain also, in 08/09 for Madrid and in 14/15 for Napoli. Maybe other seasons in Italy too - I'm only very familiar with that spectacular season at Madrid.
For Napoli he scored against Juve, Milan, Inter and Fiorentina i.e. 4 of the remaining top 5 in the league, including a goal and assist in the 2-1 win vs. Juve. For Madrid he did similarly, scoring against Barcelona, Sevilla, Valencia, Bilbao and Villareal. Scored some brilliant, decisive goals that year too, just like van Persie.
They both have proven records of scoring against the best teams in the league, in multiple seasons. It's only PL fans who don't recognise that's true about Higuain, in the same way it wasn't recognised about Zlatan until that game against England. It's a narrative not a representation if the facts.
On the other hand, van Persie let us down in the CL in the same way Higuain frequently has. Some people will debate which is more important, but you can't apply different rules to the same scenario. Either they are both bottlers on the big stage, or both elite strikers with a patchy record outside the league.