In sealed format can a non champion unit become a chosen champion

No. In sealed, a non-Champion unit can never be your Chosen Champion — the format only relaxes the tag-matching rule, not the Champion supertype requirement.

Sealed lets you choose any Champion Unit in your domain identity as your Chosen Champion, "even one that does not have a champion tag that matches its Champion Legend" (Tournament Rules 602.4.a.3.a). That waives the tag match, but the card must still be a Champion Unit. CR 103.2.a.2 makes this explicit: signature units like Tibbers (tag Annie, but not a champion unit) cannot be your Chosen Champion even if your Legend matches — and 103.2.d.3 confirms Signature cards can never occupy the Champion Zone. The same is true in Draft (TR 602.4.b.4.a). If you open no legal Champion unit, you may simply play without a Chosen Champion (TR 602.4.a.5).