And where is your proof that ANYONE said he was in the episode 'The Invisible Enemy'? Also, Marko looks NOTHING LIKE Ben Gazzara (other than hair color)!

https://www.imdb.com/title/tt0667840/

Rumors happen all the time-- rumours similar to this. Someone thought Robert Browne ( Lazarus of Star Trek) played Capt. Koloth's assistant in 'Trouble with Tribbles' ( the latter part was actually played by lookalike Michael Pataki-- God, he looked like Brown in a beard for sure). YouTube is where I read all this.

Makes sense that Marko and Gazzara be mistaken for each other. They had similar builds too. I think they were the same ancestral nationality which if so would understandably cause them to further resemble each other. Similar behavioral acting style too. One could have imagined Gazzara playing all Marko's parts just like the latter did.