The list of available branch build strategies for organizations always contains all strategies, independently of the SCMSource and / or project type.

This is because the list of available strategies is loaded like this :

<j:set var="descriptors" value="${descriptor.getPropertyType(instance,'buildStrategies').applicableItemDescriptors}"/>

Which bypasses all the checks performed by BranchBuildStrategyDescriptor.all(project, sourceDescriptor).