To paraphrase what you said: "I didn't take [statement A] seriously until [statement B]."
statement A = EC2 was owned
statement B = engineers at Amazon forbidden from using AWS
Perhaps English isn't your first language, but the way you've phrased it, you're relying on statement B as evidence/proof of statement A, directly implying a connection between the two. It's difficult to read it any other way.
Rewording your original comment: "It was only when that I heard that engineers at Amazon were forbidden from using AWS that I took seriously the comment that EC2 was owned."
statement A = EC2 was owned statement B = engineers at Amazon forbidden from using AWS
Perhaps English isn't your first language, but the way you've phrased it, you're relying on statement B as evidence/proof of statement A, directly implying a connection between the two. It's difficult to read it any other way.
Rewording your original comment: "It was only when that I heard that engineers at Amazon were forbidden from using AWS that I took seriously the comment that EC2 was owned."