First of all programs written in curly-brace languages still also have indentation to indicate statement grouping / blocks / scope, even if it's not required, so for a correct program (and that's not deliberately obfuscated), and one that's in the process of being written by an LLM, any advantage there disappears. Furthermore, having both indentation and explicit block markers provides redundancy which could be a significant advantage for an LLM (it being a probabilistic text / program generator). And for an incorrect program that redundancy is a big advantage for the LLM because it should be very easy for it to notice a mismatch of indentation and braces.
The only downside would be a very slightly higher token cost for the redundancy. I realize that Python comes out on or near the top in most of the comparisons in the linked article, but I doubt that's the reason.
The closing } ) ] token has to be generated to match exactly the beginning { ( [ many lines before. You can challenge yourself writing Lisp by hand without cursor moving backwards, and try close correctly by counting ))))))) you'd have a big headache.
A long, nested sub-routine with many () {} will cost LLM's context and makes it underperform, because the attention head have to track the state. On the other hand the indentation level can be infered as a single token[1] and saves reasoning effort.
Note these discussion is about "code generation", not parsing.
1: https://platform.openai.com/tokenizer Try input many spaces.