From the times I have hunted in the Great Basin and Montana, even on sites where Obsidian is the bulk of what is found, thumb scrapers tend to be made of harder materials like agates. The problem with obsidian is that it flakes too easily and therefore doesn't make a great classic thumb scraper. You get better results with a longer edge at a shallower angle, or with a simply larger version 3 or 4 times the size of a normal thumb scraper. And that's typically what you see on obsidian sites. Lots of reworked points that show scraping marks or knives that show scraping marks along the side, or large round disk/core shaped things, or even core bladelets and core flakes that were probably used and discarded pretty quickly
Not sure what your piece is there, it could be a scraper, but it would last about 2 minutes scraping a hide before that leading edge was rounded smooth.