The warning is genuine.
Request_Portfolio_Review__c: Marketo type 'string', expected 'boolean' (SFDC type 'boolean')
Any value entered in Marketo via forms, API or manually, would automatically set Request_Portfolio_Review__c to true (box checked) regardless the values.
You can map Marketo string to SFDC textarea.
Boolean accepts 1 or 0 (true or false), unsuitable to map to strings (up to 255 characters).