[arde]

The collective "intelligence and character of the masses" was a topic of the highest relevance back in 1939, particularly to a Jew like Einstein, while the average of its individuals less so (and of more interest to the racist ideologies he opposed). And this is not a metaphor: it is literally what is said in the note. On the contrary, it's a stretch to interpret it as a reference to the individuals in the masses instead of the masses themselves.