My guess would be the response text is passed through a rudimentary templating engine that looks for { and }. Somehow it must be processing the whole chat history. The templater fails at the unexpected braces in the code block and then just gives up (probably a try-catch ignores the error and sends the message anyway).

I don't think the code is doing anything, it looks like it might be the brackets.

That effectively the spam script has like a greedy template matcher that is trying to template the user message with the brackets and either (a) chokes on an exception so that the rest is spit out with no templating processor, or (b) completes so that it doesn't apply templating to the other side of the conversation.

So { a :'b'} might work instead.