PAA- the shitei giborim prob. only knew of murex based on the sifrei umos haolam. Therefore he must have translated that hyacynth, which is recorded to have been dyed from the murex, to be techeiles.
Holymoe- I can confirm the story. I heard it from one of the avreichim involved.
As for bal tosif which was brought up in the other thread, i am willing to argue that point