When coworker sees Atsushi walking through the crowd with Ibuki and Hiiro, look at how much Ibuki has grown. The coworker has also aged, but not Atsushi (or Hiiro, obviously) which is why the coworker did not believe it could be him.
More story showing how their relationship evolved would have been awesome but I’m good with this ending.
Eien no Yoru no Hate