The part of the comment I was responding to was talking about it being impractical to have that many containers running, which isn't true. Later I mentioned that there are probably other benefits (having everything in one process does remove the need for TLB flushes and so on), but that isn't the only thing they said. They didn't say it's drastically faster, they said it's "unrealistic" which simply isn't true.

Also things like page cache sharing would make a huge difference to the real memory cost of having many processes. When you run 10k programs you don't have 10k copies of glibc.