When you mark a Microsoft 365 threat as ‘Remediated’ in Defense.com it will automatically update this threat in your Microsoft 365 account. You will see this in your Microsoft account as 'remediated by a third party'.
Conversely, once you have made the necessary changes and remediated issues in Microsoft 365, this will automatically update in your Defense.com account, too. Any configuration changes in Microsoft 365 that affect your Microsoft Secure Score will also be automatically reflected in your Defense.com account.