Not really. The point is that SB winning coaches almost universally have some level of success in their first 8 years, even those who won a SB after Year 8. Take Cowher. Yes, he won his SB well after his 8th season. But through 8 seasons, he had given the Steelers enough on the field success to make it plausible he could eventually win one. Cowher, by the end of his 8th season, had already coached in a SB and another AFC Champ game.
In other words, if Jason Garrett had coached in a SB and another NFC Title game by this point, we'd all be having a much different discussion about his chances of winning a SB.
You seemingly forget that part of the analysis. That those coaches who had long tenures (and eventually won or still never won a SB), all had way more success as a coach by Year 8 than Garrett to justify their continued employment. Take Reid. He coached in 4 NFC Champ games and a SB by Year 8. Marv Levy is another coach who coached a long time and never won a SB. By Year 8 in Buffalo, he had coached in 5 AFC Champ games, and 4 SBs.