It certainly means API changes are not anticipated, and that there should be a very significant problem with an API to lead to an API change at that point; certainly one would hope that fundamental flaws in an API that make it unsuitable for release would be identified well before an RC.

But its certainly possible to have reasons to make a breaking change from an RC (e.g. -- not necessarily relevant to a front-end framework, obviously -- a security vulnerability uncovered that reveals a critical flaw in the logical design, rather than implementation, of an API.)