The activation link in ArcGIS for Developers does not indicate users cannot activate the account with an existing public account.
Last Published: January 21, 2015ArcGIS Online
Bug ID Number
December 30, 2014
July 8, 2022
In Product Plan
The issue has been added to the development team's backlog. The goal for issues with this status is to address them in a near-term release (next release or two). Check the issue's Additional Information or Alternate Solution section to see if there is helpful information until it can be fully addressed.
Create a new account when activating a trial account.