I think if you read the entire section it becomes pretty clear. This is not specific to Fleet managed instances but rather on-premises instances that are managed by Systems Manager regardless of use (fleet or otherwise).

On-Premises Instance Management

You can set your account- and Region-level settings for on-premises instances to either standard or advanced. With standard on-premises instances, you will be able to register up to 1,000 on-premises instances per account per Region at no additional charge.

If you need to register a larger number of on-premises instances, you can change your account- and Region-level settings for on-premises instances to advanced. This will convert all the existing instances in the account and Region from standard to “advanced.” With advanced on-premises instances, you will also be able to use Systems Manager Session Manager to interactively access your on-premises instances. With this setting, you will also be able to use Systems Manager Patch Manager to patch Microsoft applications hosted on on-premises instances.

Systems Manager advanced instances are priced on a pay-as-you-go basis. You are charged based on the number of advanced instances activated as Systems Manager managed instances and the hours those instances are running. Charges are not incurred for the hours where an advanced on-premises instance is de-registered, shut down, or terminated for the entire hour. This pricing applies to instances (on-premises, other cloud providers, or Amazon EC2) registered using Systems Manager activations.

Visit the AWS Systems Manager documentation or product page to learn more about advanced on-premises instance management.

Pricing

Pricing example
You have 500 on-premises instances registered in account A with standard on-premises instance management setting and 1,500 on-premises instances registered in account B for 10 days with advanced on-premises instance management setting.

Cost of 500 instances on standard on-prem instance management in account A = No charge

Cost of 1,500 instances on advanced on-prem instance management in account B = 1,500 * $0.00695 * 10 * 24 = $2,502

Total monthly cost for account A and B would be $2,502