The object-cache.php installation is designed to be compatible with any existing caching layer. In particular, if you have an existing object-cache.php it will copy that one and then continue to load it after the Performance Lab’s version is installed: https://github.com/WordPress/performance/blob/01504c5493ef71e355ac6e9f5165e938729c46cc/plugins/performance-lab/includes/server-timing/object-cache.copy.php#L76-L86
Is this not working for you?
That said, you can disable it from being installed by adding define( 'PERFLAB_DISABLE_OBJECT_CACHE_DROPIN', true) to your wp-config.php.
In any case, the use of object-cache.php as a means to start early timing is a hack. It would be great if we could use a different mechanism to start timing early, like installing an mu-plugin.
