Mania shows the Ruby as being used to travel across time and space, as a power source, and potentially to create illusions.
Forces shows the Ruby as being used to create illusionary copies of certain individuals, as a power up for Infinite, and by Infinite to create illusionary versions of locations where it's used.
Given that we know the Ruby is involved in Mania, Eggman has the Ruby in his possession during those specific events in Mania, and the Ruby is specifically being used before, during, and after Stardust Speedway, there would need to be some sort of outside context given for us to believe Metal is the real deal. We only ever see him present within Stardust Speedway, so the implication seems to be that either he's native to the time in which Sonic and the others were sent, or the Ruby was used to create an illusion of him. No outside context for the Metal native to Sonic's time having been sent there is given, and the fact that he appears in Stardust Speedway specifically only makes it more likely to not be him.
If Chaotix happens before Mania, then Metal shouldn't even be on Little Planet. If Chaotix doesn't happen before Mania, Metal should still be a broken heap of junk.