Important to keep in mind here that all sales numbers for digital titles are, by necessity, estimates because major marketplaces like Steam not only don't release numbers but place limits on what kind of information developers can release. Previous services for estimating sales numbers were shut down by Valve. SteamSpy was at least fairly accurate up until it was shut down, but it also leaves out sales on consoles and non-steam storefronts.

Another factor is that many of these games are 'prestige brands' like Minecraft or The Walking Dead, which means there may be partnership funding involved. This is demonstrated by how Telltale spoke about keeping a skeleton crew around to finish a game in order to fulfill obligations to partners - if that's happening it's because money changed hands.

(I agree in general with the premise that the writing has been on the wall for Telltale, just not that a steamspy chart is proof that the company was already dead in the water)