Karkaroff is cowardly, and Snape has “left him forever”.
Snape had a Dark Mark, so he was definitely part of the Death Eater “inner circle”. He would have been expected to be there, and Voldemort calls out gaps – for example, the Lestranges – so Snape would get a mention somewhere, and this seems to be it.
In Half-Blood Prince chapter 2, Snape gets challenged by Bellatrix about his loyalty, and his apparent lateness at his rebirth:
“The Dark Lord's initial displeasure at my lateness vanished entirely, I assure you, when I explained that I remained faithful, although Dumbledore thought I was his man. Yes, the Dark Lord thought that I had left him forever, but he was wrong.”
Although Snape obviously wasn’t there to hear what Voldemort said, the wording still fits.
A lot of the chapter involves him justifying himself to Bellatrix, and explaining that his late return was to remain as a spy within Hogwarts. This includes explaining why he never sought to find Voldemort, and why he fought against him when he tried to get the philosopher’s stone.
As for Karkaroff, remember that Barty Crouch Jr. will have told Voldemort of how he testified against other Death Eaters, and has seen him growing visibly uncomfortable over the last year at Hogwarts. The description “cowardly” fits for Karkaroff, not Snape.