It's very KHL, which IS NOT a development league.
Normally you don't see many players under 20 start there, Tyipically they pay their dues, MHL to VHL to KHL bench to KHL starter.
My suspicion is it's about pecking order, sitting a veteran for a 18 year old would not sit well with teammates, but if Michkov is as good as advertised, in a month or two he'll work his way up to 1C b/c he's the best player on the team and the HC wants to win, and so does his teammates.
And from the Flyer's perspective, limiting his PT at 18 is good b/c it'll reduce his chance of serious injury while he physically matures.