I did manage to make it more compact by turning that function into a private val, like so:

// Declaration
private val fileFilter = object : AbstractFunction1<Tuple2<String, PortableDataStream>, Any>() {
        override fun apply(tuple: Tuple2<String, PortableDataStream>?): Any {
            return true
        }
    }
// Call
sc.binaryFiles(path, 1).filter(fileFilter)