Thank you for your well formulated contribution to this answer.
Few years back I did experience this issue as well, and I notice that squid swapping was making the IO going up as hell.
After tuning up squid on "what should be cached" and what should not, I reduce the cache size.
Doing so, I've seen a significant performance improvement.
Tuning squid cache is not easy but you'll get much better performances at the end.
But you are right with one point here, I do not have enough information to say that hack back shouldn't use SWAP. Maybe he got some SSD, in that case it might not heart too much (although I never tried this configuration).
But at the end of the day (in my opinion "and by experience", and it seems that by the squid team as well) SWAP usage should be use with caution as performances WILL suffer. (see my quote below)
If you have a low memory server, and a large disk, then you will not necessarily be able to use all the disk space, since as the cache fills the memory available will be insufficient, forcing Squid to swap out memory and affecting performance. A very large cache_dir total and insufficient physical RAM + Swap could cause Squid to stop functioning completely. The solution for larger caches is to get more physical RAM; allocating more to Squid via cache_mem will not help.