A question of the xss vulnerability's Proof of concept

[supercodeing35271 supercodeing35271 <supercodeing35271 () gmail com>]

Hi,i have a rookie's puzzle of some xss vulnerability bug report.
If there is a vulnerability Poc in a report just like this:
http://www.example.com/index.php?id=<img src=g
onerror=alert(document.cookie)> or
http://www.example.com/index.php?id=<img src=g onerror=alert("xss")>

OK,this may cause a alert when run the url.But my question is that why
it can say this:
"Vulnerability Details:
User can execute arbitrary JavaScript code within the vulnerable application."

I see many report have the same words.Why a alert could be output then
we just can say "User can execute arbitrary JavaScript code within the
vulnerable application"?
The last is that i am a rookie,seems that this is a stupid ask but it
is a true puzzle on me.

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