No it's not. He sets the scope as "all capitalist nations that have not been imperialised", which is logical. How can an imperialised country be imperialist towards another?

You are trying to include them in your argument.

The claim was

It’s a trend observable in all capitalist nations. If you develop enough, the rate of profit falls, and so you need to expand outward to profit. This is the basis of imperialism, the carving out of the global south for profit. Across the west, this is a fact, even if it manifests in different ways.

The trend is observable on the imperialized nations as well as the imperialist ones.

Imperialism is not a one way street, the effects of imperalialism are observable (lower capitalistic development, higher profit extraction, etc).

The fact that the countries with more developed capital are the ones doing imperialism and the countries with less developed capital are the ones imperialized (and oberving how this stays true historicllly) is proof of the trend.