Not completely true, but mostly accurate. You can get around it though. "attacker" is an exception and is passed to the timers to facilitate timing multiple mobs that use the same attack. It's done this way specifically to allow multiple instances of the exact same timer to exist from different sources. You can abuse this to your advantage by capturing your JCap target and naming it 'attacker'.
So if you use:
/tellchannel uyaemscapchannel JCap on Durancer
And capture it with:
You tell uyaemscapchannel \(\d+\), ?"JCap on (?<attacker>[\w\S]+)"
...then you can trigger the same timer over and over, and a new timer will be created for each person and the name of the 'attacker' (in this case the player you JCap'd) will be appended to the timer each time you tag someone else with it.
<edit>
To expand on the idea, in case I'm not being clear, (which, honestly, I rarely am)... If you're using Profit and you have something like...
<Data Name="Button1" Macro="SpellForMyLevel="Jester's Cap"
useabilityonplayer Parent.Target SpellForMyLevel
tellchannel uyaemscapchannel Jcap on Parent.Target" Tooltip="(65) Jester's Cap"/>
in your quick raid buttons, then what you're capturing with [\w\S]+ is always going to be a player's name. Since you're naming the captured pattern 'attacker', it will be passed to the timer.
You can use one universal trigger to trigger a single timer, and the timer will create multiples of itself as long as the 'attacker' data are different.
So you stick:
You tell uyaemscapchannel \(\d+\), ?"JCap on (?<attacker>[\w\S]+)"
as the regexp for your trigger. Have it trigger a timer named JCapTimer or whatever. If you JCap Blah, then you'll have:
JCapTimer
2:00
Because the first instance of a timer is never named. But if you then JCap Blubb 30 seconds later, your timer window will change to
JCapTimer - blah
1:30
JCapTimer - blubb
2:00
Anyway. One trigger, one timer. It can be done -- whether it should be done or not is a separate issue.
If you're using the same phrase in your direct target hotbar macros, it will work for them as well, giving you a fairly universal solution to the problem, especially if your raid force changes a lot. It saves you from having to clone a trigger and a timer every time a person you want to cap regularly changes.
If I'm still not making sense, let me know and I'll try to elaborate further.