The argument seems to rely on measurement data. That is, all the intelligence measures we have are correlated, therefore there is a single intelligence factor. What this argument misses is the fact that not observing something doesn’t mean that it doesn’t exist. For example, we weren’t able to observe atoms for a long time time but they were there all along. Gardener tried to make this argument in the 70s regarding multiple intelligences but I don’t know about the measurement evidence backing that theory up.