I agree. There is some rule about him needing to play the last 6 games this season though.
That rule is tied to when his contract expires. It doesn't have anything to do with accruing seasons for free agent status.
The best way to look at it is this.
To be an unrestricted free agent, two things need to happen.
1) Your contract has to obviously expire
2) You have logged 4 years of NFL service (if you only have 3 years of service, you are a restricted FA).
The Aug 6th date is tied to accruing seasons towards that 4 years of NFL service (#2 above). The Week 10 date is tied to whether or not the season counts as a season played under the contract (tied to #1 above). In other words, if he doesn't show up by Week 10 this year, his contract doesn't expire after the 2020 season, it expires after the 2021 season, regardless of whether or not he eventually logged that 4th year of NFL service.
So let's say Elliott doesn't try to show up until the 14th game. This season doesn't count towards his NFL service years AND it doesn't count as a season played under his contract. Now let's say he shows up before Aug 6th next year and plays the 2020 season. In that case, he would have met the 4 years NFL service requirement to be an unrestricted free agent, however his contract wouldn't expire after the 2020 season. It would expire after the 2021 season. So by not showing up by Week 10 this year, he essentially moves the date he can become a free agent from after 2020, to after 2021.