I'm trying to figure out what you feel was wrong with the situation, prompting you wonder about filing an ASRS?? You were changing altitudes, which is correct. ATC provided you a courtesy traffic advisory, which is correct. You took heed to the courtesy traffic advisory, which is correct. ATC politely asked you to change altitudes, which is correct.
I see nothing that would be unusual. Chances are the traffic was not a factor or he would have said so. It's just an advisory of nearby traffic. You can always ask the for the distance or if it is a factor if unsure.
The confusion caused by the altitude reprimand in this situation caused the pilot to think he had done something wrong. This could lead to a situation where in the future a different pilot receiving the same reprimand may decide that reaching the altitude took precedence over the traffic advisory.
The result of this ASRS report may be to add into a controller advisory memo that after such advisory to inform the pilot they are clear of traffic for continued descent.
What I saw missing from the OP is the call, "traffic no longer a factor" before the admonition on being at the incorrect altitude after a traffic below conflict call. The guy is not reporting his own error, he made none. He pointed out a controller or communications error that is proper to address as to the intent of the reporting system.