Unless you choose to opt-in, your campaign will be automatically opted-out of Microsoft Search Partner traffic.

This statement is false. By default, campaigns in Microsoft Advertising are opted-in to Microsoft Search Partner traffic. This means your ads will automatically appear on search partner websites in addition to Bing and other Microsoft sites, unless you actively choose to opt-out. Opting out requires a specific action within your campaign settings. Therefore, the premise that you must opt-in to receive this traffic is incorrect.