... there are some very modern items there, such as the Salt Creature ...
Why assume that's modern? The M-113 creatures were extinct by the time "The Squire of Gothos" took place, their planet a decaying ruin.
Extinct? Maybe not, Sulu and other guy were suspended in time apparently, so too could have the Salt Vampire. It was the actually "last one."
Now did Trelane go to M113 to collect the one we saw? Or were the Salt Vampire's at one point a star fairing species who ran across Trelane planet in the past? The one we saw being the last survivor of that crew.