Does bitcoind ERROR "Disk space is too low!" refer to RAM or actual harddrive?
It refers to actual disk space. RAM is memory, not storage. The term "disk space" is very specific.


  Then a while later this:

[2020-01-20T09:05:59Z Reindexing finished]

  
  But bitcoind continued grinding on... Well then what is all this? ::


Your node was not fully synced, so after it finished reindexing the blocks on disk, it continued to download blocks from its peers and finish syncing.