Why implementation gradle dependecies are exposed to consumers - gradle

Question

The java-library plugin documentation says:
Dependencies appearing in the api configurations will be transitively exposed to consumers of the library, and as such will appear on the compile classpath of consumers.
Dependencies found in the implementation configuration will, on the other hand, not be exposed to consumers, and therefore not leak into the consumers' compile classpath
However, it doesn't work for me. I see that the implementation dependencies are exposed to consumers too.
Here one example:
project_a -> build.gradle
...
dependencies {
// Dependency supposedly not exposed to consumers in their own classpath compilation
implementation 'com.google.guava:guava:23.0'
}
...
project_b -> build.gradle
...
dependencies {
implementation 'my-company:project_a:1.0'
}
...
I expected that guava doesn't appear in the project_b' classpath. However I see guava and all their dependencies on the project_b compile classpath.
project_b>> gradlew dependencies:
...
compileClasspath - Compile classpath for source set 'main'.
\--- my-company:project_a:1.0
\--- com.google.guava:guava:23.0
+--- com.google.code.findbugs:jsr305:1.3.9
+--- com.google.errorprone:error_prone_annotations:2.0.18
+--- com.google.j2objc:j2objc-annotations:1.1
\--- org.codehaus.mojo:animal-sniffer-annotations:1.14
...

Answer 1

The reason you do not experience this behaviour is because your dependency goes through a Maven POM.
The separation of the Maven compile and runtime scopes is only supported as of Gradle 4.6 and is currently behind a feature flag.
In short, if you add the following to settings.gradle of project_b, you will get the expected behaviour:
enableFeaturePreview("IMPROVED_POM_SUPPORT")

Answer 2

I believe the documentation refers to multi-project builds, and that a consumer is considered to be a project (not a jar) that depends on another project in the build.
For example, consider this project_b/build.gradle:
dependencies {
implementation project(':project_a')
}
With this (and the appropriate settings.gradle), I can observe the compile classpath changes as the doc describes (for use of api versus implementation). I have placed a working example here.