Yes, because a few preseason goals is more important than the previous season where he was marginal AHLer? Seriously..
Because 13-17-30p in 48gp before age 21 in the AHL during his D+3 season is somehow marginal? Jesus.
Most people forget that Kostin wouldn't have played in the AHL in his D+1 had he been north American, because he would have been ineligible - he would have had to play NCAA or CHL hockey. Most North Americans play their D+2 as an overager season after their up to 9 game cup of coffee in the NHL before being sent back to juniors because THEY ARE AHL INELIGIBLE until age 20. It's NHL or juniors, so your list of comparables playing in the AHL at 18 and 19 is basically non existant...
Here's a list of some players off the top of my head who are having successful NHL careers who played in the AHL before. Some stars, some complimentary pieces - look at their D+3 seasons and compare them to Kostin's...
Kostin's D+3
2019-20 NHL 1-0-1 in 4gp
2019-20 AHL 13-17-30p in 48gp
AHL PPG 0.625
Kyrou's D+3
2018-19 NHL 1-2-3p in 16gp
2018-19 AHL 17-26-43p in 47gp
AHL PPG 0.915
Gurianov's D+3 (#12 OA)
2017-18 AHL 19-15-34p in 74 gp
AHL PPG 0.459
Schwartz's D+3 season (#14 OA)
2012-13 NHL 6-7-13p 45gp
2012-13 AHL 9-10-19p 33gp.
AHL PPG 0.576
Sanford's D+4 season (D+3 in NCAA)
2016-17 NHL 4-4-8p in 44gp
2016-17 AHL 11-5-16p in 25gp
AHL PPG 0.64
Stone's D+3 season
2012-13 NHL 0p 4gp
2012-13 AHL 15-23-38p in 54gp
AHL PPG 0.704
Couture's D+3 season (#9 OA)
2009-10 NHL 5-4-9p in 25gp
2009-10 AHL 20-33-53p in 42gp
AHL PPG 1.262
Kucherov's D+3 season
2013-14 NHL 9-9-18p in 52gp
2013-14 AHL 13-11-24p in 17gp
AHL PPG 1.412
Tell me again he's a bust? Again he's 21 and drafted outside the top 10... He's playing his D+4 season in a pandemic, after getting the end of his D+3 season where stats get padded removed.