3 games for a total of 12 minutes of ice time in those games combined. Yet that some how makes him more ready for the NHL as a full time player than someone who has 5-6 Pro years under his belt.
You are also forgetting the entire season he had playing in the Stars' AHL system, and that he had two pro training camps (and who knows what else) to get himself prepared to play in the NHL. Given that he was an OHL player, I'm guessing that he spent the offseason learning and developing with the Stars' before being sent back to the OHL. In fact, he tweeted that in 2017 and in 2018. So, the Stars' had a huge hand in Robertson's development since he was drafted.
You also have the fact that Robertson got to play on North American ice for most of his career, which many experts have stated is a determent to many international players coming over as they have to adapt to the smaller ice size. Robertson also speaks English and is accustomed to North America culture.
If it was so easy for 'pro' players in the KHL to translate their game to the NHL, then guys like Gusev (who were stars in the KHL) should become stars in the NHL. Or the equivalent. Gusev, Shipachyov, Lehtonen, Barabanov, and even to the extent that Tolvanen struggled a lot to adapt to the NHL/AHL.
Here are some players that are in the top 10 of scoring in the KHL:
Stephane De Costa (career high in 18 goals and 58 points in the AHL)
Brian O'Neill (career high is 26 goals and 80 points in the AHL)
Markus Granlund (played in the NHL for a few seasons)
Reid Boucher (again played in the NHL for a few seasons)
Jori Lehtera (again played in the NHL for a few seasons)
Being a star in the KHL doesn't mean at all that the player is going to be a star in the NHL. In fact, more players struggle to translate their game from the KHL to the NHL - which is why Gusev is on his fourth NHL team, Shipachyov is back in Russia and Barabanov has been traded.
Finally, no one said that Kaprizov wasn't going to be a star in the NHL. The fact that his game adapted so smoothly and almost effortlessly between the two leagues is massive. A lot of experts cautioned that Kaprizov's first season or even two may be rough. Everyone hoped (and that's the keyword here) that he'd be the offensive catalyst for Minnesota, but were relying more on Fiala to be the offensive spark. The fact that Kaprizov is 10th in goals is astounding for a young player in his first season in the NHL. When Panarin came over, he was 20th in goals. Kaprizov is on pace for 39 goals in his rookie season. Even for someone that is 23-24 year's old that is astounding. Not a lot of players can do that in their first NHL season, regardless of how old they are. Think about it; how many NCAA seniors translate that smoothly from the NCCA to the NHL? How many free agents signed from International come over and score 39? Even Panarin couldn't do that with Kane stapled to his other wing.