That engineer is Senior in title only. Unfortunately this is a common pattern - a mid-level developer will play "hero", make a mess that only they understand, and - if given the title/responsibility by immature management - proceed to royally screw their team.

Actual Seniors have the experience and foresight to handle this properly. Either build code with enough documentation/tests that others can understand it, or make an onboarding/mentoring plan for team mates, or estimate based on a teammate doing the work.

In this position: the engineering manager should remove the fake Senior from the team, that will force them to get up to speed.