In a talk last year by Luis Elizondo, this slide was shown
The one that caught my eye was the second from the right
Which instantly reminded me of a couple of pieces I have on my bookcase:
These are the result of burning some thermite in a flowerpot.
Even the shape is reminiscent of the bottom of a flower-pot, which is a common vessel for burning thermite (relative heat resistant, at least not melting). After chipping away the cracked piot you get something like this:
The bottom part is iron, the upper part with more bubbles is slag, a mixture of iron, iron oxide, and aluminum oxide. When sawn in half vertically you get the cross-sections like in my photos above, and in the TTSA image.
Given the visual similarity, I'd say it's pretty likely that this is what their sample is. Which raises the question of where they got it from. A thermite residue puck is not something you find laying around, unless you made it yourself.