I'm not 100% sure, but I think Bob might be wrong there. Andrew Copp signed an ELC and burned a year at the end of 14-15 season, and Winnipeg had to protect him. I believe Mittelstadt, and all the other 20 year old players who have signed in these past couple days and will burn a year off their ELCs here, are in the same boat, unless there's some weird additional age considerations.
But Tolvanen is different because he's younger, unlike all these other guys he doesn't turn 20 this calendar year. So I think he does need 10 or more games to count this as a pro year. Similar to Reinhart, who played 9 NHL games in 14-15 and didn't have to be protected. Bennett played 12 games, and had to be protected.
Copp had played 15-16 and 16-17 in the NHL prior the Draft, while only one game in 14-15. But you might be right about Bob being wrong, because Casey will by then have similarly played 18-19 and 19-20 before the 2020 XD.
But, then Tolvanen also will have played two seasons (assuming they stay up, of course) by 2020.
Sliding/burning a year off ELC is different from what it considered a played season under XD rules though, because Slide eligibility is age-related whereas the XD eligibility rules are not.
Actually, looking it up, the Vegas XD rules said that:
"All first- and second-year professionals, as well as all unsigned draft choices, will be exempt from selection (and will not be counted toward their club's applicable protection limits)."
There was some number about games played, I'm not sure it was ten. Also, a year in AHL is a professional season.