Depends on how the extension works probably. Had the issue with some extension but the Qwant extension for example doesn't leak - you can check yourself when capturing the http(s) traffic with something like proxyman.
Interesting, something must have changed then - the qwant search extension definitely does not leak the search to the default search engine - I checked myself with Proxyman.
I don't know your setup but in my opinion something like proxyman / mitmproxy or even Wireshark would be the better way to analyse this.
I tested quite often and never saw the leak with Qwant.
I have Proxyman and Wireshark. IIRC I used Proxyman also when writing my blog post.
In any case, Little Snitch is not lying and inventing packets. I don't know why you aren't seeing anything, but as I said, it's a race condition, so that can happen, and in any case, I would say that it's a better way to analyze this by adding Little Snitch to your setup.
I quickly downloaded the kagi extension for testing and there I can see the leaking - the Qwant extension still does not leak. Maybe you could also test the Qwant extension?