I'm not sure what is meant by "exact" here - do they describe their binarisation process at all? This seems more like an XOR benchmark than a rag benchmark, no mention of recall or other relevant performance metrics
Some (not all) of your questions may be answered by the linked article near the top of the submitted article, which goes into more detail about how much is lost quantizing to 1 bit (and 1 byte): https://huggingface.co/blog/embedding-quantization
exact in this case means that all the vectors are compared against the query vector. Where as other search methods such as HNSW are approximate searches.