I think the point is that both Raikkonen and Verstappen were top of the world in international karting, which was used a lot more back then (cost reasons? apparently it’s very expensive now, seems they get out of it asap to cheap single seaters. Jon Lancaster’s tweet of a motor-home at a kart event backs this up!). So being top in karts for a few years, then a quick transition through single seaters back up to F1 level, which karts are closer to than slow single seaters, was the usual thing then (Alonso, Button etc.). So, overall they would have more fast speed experience than those young drivers today – Sirotkin, Chilton, Pic etc. who haven’t been right at the front of every championship they’ve done in the slower lower levels, compared to those guys in international karts and then e.g. Kimi dominating Formula Renault 2.0.
But there’s no doubt Sirotkin has talent (top 10 in FR3.5 standings so young, after doing well in the lower levels, keeping up on pace). He might be coming up a year early for ideal development, and skipping GP2, but if you can develop securely in F1, why not do so. His pace may flow in a few years, like Chilton, and he probably has some potential driver development left to go, being so young. But at the moment he seems more a Bianchi than a Chilton (fast adapter), but hopefully is not going to do an Alguersuari by coming in too young.