I guess that is a bit of a bug... It is possible to give any unit the Engineer capabilities, but the Colonist already has that. So I think that only has to do with their double speed.
Transforming is linked to the Explosives tech so I guess they forgot to add that as an engineer capability as well.
Wait a minute... Colonists CAN transform, but there is a bug of an entirely different kind. The transform menu option is disabled, but pressing "o" lets them transform anyway.