Unable to retrieve Email FullContents via API for certain Activities

Via the Rest API Bulk Activity export, I have pulled a list of Activities that were performed within the last few months. Each Activity has a value in the "primaryattributevalueid" field which I assume ties to the ID from the Email API. Now, I need to get some more info for those Activities, such as the actual email HTML.

I have also pulled (what I believe) is a full list of emails via the Browse method:
123-ABC-456.mktorest.com/rest/asset/v1/emails.json?maxReturn=200

 ( this returns a list of ~150 emails; well under the 200 limit so need to deal with page offsets).

My problem is that I am seeing a lot of Activities with primaryattributevalueid that do NOT have corresponding email ID's from the email API. I believe this can be attributed to the following warning on the API docs here - https://developers.marketo.com/rest-api/assets/emails/#query  which states that emails that were part of Testing aren't returned by the the 'browse' API.

Assuming everything stated above is accurate, I believe I still should have a method of getting the contents of the Email via the FullContents method.  For example:

"123-ABC-456.mktorest.com/rest/asset/v1/email/2335/fullContent.json"  SHOULD return the contents for activity 2335. But instead, I get the following API response:
"errors":[{"code":"702","message":"Email not found"}]}; 

Any thoughts on what I need to fix, or alternate methods for pulling Email Content data to attach it to Activities?