I’m not sure where this belief came from, or why the people who believe it feel so strongly about it, but this is not generally true.

With the exception of GPL derivatives, most popular licenses such as MIT already include provisions allowing you to relicense or create derivative works as desired. So even if you follow the supposed norm that without an explicit license agreement all open source contributions should be understood to be licensed by contributors under the same terms as the license of the project, this would still allow the project owners to “rug pull” (create a fork under another license) using those contributions.

But given that Zed appears to make their source available under the Apache 2.0 license, the GPL exception wouldn’t apply.