It's not so much that it is not possible to address any given issue in a different way. E.g. they could have tested the soap dispenser with different skin tones

It's not so much that it is not possible to address any given issue in a different way. E.g. they could have tested the soap dispenser with different skin tones. Rather the issue is that a team is more likely to realize issues caused by interaction between a product and various groups if those groups are part of the team working on the project. It's a discoverability issue.