A permissions boundary limits the permissions an identity policy can grant. It is a ceiling, not a permission grant. For the identity-policy path modeled here, the request needs an applicable allow and must also fit the boundary. An explicit deny still overrides the result. AWS has additional resource-policy and principal-type rules, so don't generalize this exercise into a complete authorization model.
Role trust answers who can assume a role. A permission policy answers what a resulting session may do. Review both, especially for cross-account access, and distinguish the trusted account or service from a wildcard principal. The exercises make the boundary visible and then add an identity policy. If you remove the boundary just to make a test pass, you've changed the safety requirement rather than repaired the missing grant.