Iterate through all inner blocks recursively and get navigation link block’s post IDs.

function block_core_navigation_get_post_ids( $inner_blocks ) {
	$post_ids = array_map( 'block_core_navigation_from_block_get_post_ids', iterator_to_array( $inner_blocks ) );
	return array_unique( array_merge( ...$post_ids ) );
}