I don't think this is correct—I read the earlier section of the docs ("Block Form Of Source, Git, Path, Group And Platforms") as saying that the block form is equivalent to the "source explicitly attached to the gem", the first priority item in your link

However, this section is concerning:

> The presence of a source block in a Gemfile also makes that source available as a possible global source for any other gems which do not specify explicit sources. Thus, when defining source blocks, it is recommended that you also ensure all other gems in the Gemfile are using explicit sources, either via source blocks or :source directives on individual gems.

Yikes! This is yet another easy footgun for people to reintroduce this issue