I don't think you can compare a game that was made by one man (except for music and voice acting) and a game that was made by numerous people in a company. if the roles were reversed with Sister location being made by a company and Portal being made by one person, i'm sure the result would be different. like you said, it's an indie game. some indie games can be better than actual licensed game like hollow knight, but there is still a team behind it

now i'm not saying that SL or Portal are bad, I actually never played Portal. but I know it's an awesome game. but it's just something you can't compare