Autothrottle extension doesn't alter slot concurrency in any way (it used to but it doesn't now), it only changes slot's download delay.

I think the missunderstanding with concurrency limits is because current algorithm alter delays so it follows response latency which leads to 1 concurrent request with rare increases to 2 or 3 concurrent requests when latency changes abruptly.   

Concurrency limits are imposed by the downloader itself, it is not managed by this extension at all.