My point being that drivers he could be compared alongside, having also regularly competed in F1, and commonly referred to as possible greats, succeeded or showed well across multiple disciplines.
There are dozens of other drivers who you could add, including Schumacher (Group C winner), Moss (Mille Miglia), Hawthorn, Phil Hill and Rindt (Le Mans winners) and even Hakkinen (DTM winner).
Seeing Mansell winning the Indycar title at the first attempt made me appreciate how hard it is to jump from one category to another and do well. Similarly I admired Raikkonen and Kubica for enjoying other disciplines (ultimately what curtailed the latters career from any chance of his addition to the list) and why I'm looking forward to seeing how Alonso goes this weekend.
Of course there are other drivers (AJ Foyt, Sainz, Loeb, Kristensen, Bell), but I was merely sticking to his contemporaries, though more highlighting what they had done outside of F1 rather than only what they had done in F1.
For example, I doubt many would say Nigel Mansell is the greatest F1 driver, but that he was concurrently F1 champion and Indycar champion is a greater achievement.