I agree, it makes no sense -- on the surface. But let me speculate on why it happened.
(1) The expansion draft. EVERYTHING in personnel moves this year has to be evaluated in terms of that. Siegs was draft-eligible so the team had to protect him or potentially lose him. And wouldn't a young, cheap, solid defender be just what an expansion team might be looking for? I suspect the organization has 3 other defensemen that they want to protect and didn't want to lose this particular asset for nothing.
(2) Fehevary is sitting down there in Hershey, and maybe the organization wanted to keep him stashed this year and bring him up next year. Like a lot of prospects, his development may be retarded a bit by COVID. But I suspect, even under those circumstances, he may be higher on the depth chart than Siegs, especially in the mind of Laviolette, who likes mobile defensemen.
(3) Alexeyev is now sitting in Hershey under COVID quarantine protocol, but he has already played 55 games plus 9 playoff games in the KHL this season. The Caps may know more about his development than Fehevary's this year -- I have to believe someone associated with the organization was keeping a careful eye on him in Russia. Maybe the Caps see the need to have room for both him AND Fehevary on the roster next season. And he IS a clone of Siegs, but with more mobility.