The devices themselves still run a JVM (Dalvik and ART) that's basically compatible only with ~JRE7 bytecode. There are some new features in ART since N, but still the JVM isn't fully Java 8 compatible (afaik invoke-dynamic for lambdas is still not supported).
So the Java 8 tools you're talking about essentially take Java 8 compiled bytecode and patch out the parts that aren't compatible with device JVM (e.g. they replace lambda invocations with anonymous classes like you'd use on Java 6 etc.).
This doesn't work for things like Scala which use these new features in patterns those tools can't patch/recognise.
So the Java 8 tools you're talking about essentially take Java 8 compiled bytecode and patch out the parts that aren't compatible with device JVM (e.g. they replace lambda invocations with anonymous classes like you'd use on Java 6 etc.).
This doesn't work for things like Scala which use these new features in patterns those tools can't patch/recognise.