It’s far too early to use is_page.
is_page is not available until after plugins have loaded. You would need to figure out if it’s a singular page or not via the URL.
If the goal is to avoid assets being enqueued by a plugin, the solution is to dequeue those assets, which is something that can be done from the theme or a plugin after it has loaded, but before wp_head is called.
Unhooking the action that enqueues them is also an option. Note that you should file a bug report with the plugin authors regardless of wether you find a solution, but conditionally activating the plugin is not the only or best solution to your problem