I'm not disagreeing with the assessment of Cody, but I will point out that Tech as the unknown operative seems like it would be be the more dramatic choice, where by dramatic here I mean in terms of how much direct impact there would be on the main characters. Tech was one of the Bad Batch, Cody wasn't. Plus, they think Tech is dead, whereas him fighting against them irreversibly brainwashed (if it is irreversible) might be worse than dead for them. Although we've kinda already been there with Crosshair, we haven't really, because this would be a shift in loyalty backed by brainwashing that wasn't present in Crosshair (at least as far as we know right now).
As a counterpoint, this episode featured EVERY SINGLE major named clone (who was alive last time we saw them) that has appeared in this series EXCEPT for Cody (and Cut Lawquene, but his absence makes sense in context).
Think about it. Rex, Howzer, Gregor, Scorch, and now even Wolffe to complete the Rebels trifecta, but nobody even MENTIONS Cody?
The absence is itself noteworthy. Unless the episode actually DOES include Cody and we just don't know it yet.
I would also personally consider it bad writing to have it be Tech after introducing the idea that Crosshair was immune because he was a 99. Not that writers are above being dishonest with the audience, or that Crosshair couldn't just be wrong, but such a reveal would denigrate Tech. Mark him as lesser than Crosshair in a fundamental way.
Whereas Cody is a reg, and doesn't fall under the same umbrella.
Also, not for nothing, but the Operative has shown skills more in line with Cody than Tech. Cody being both a skilled shot and an excellent knife fighter have both been established previously, where there is no indication Tech could go head to head with Crosshair and match him. Similarly, Tech would have been readily able to repair his arm computer device, while Cody likely wouldn't have the know how.
Like I said prior, I thought of Tech first and all the reasons you mention. I just don't think he fits the evidence thus far. But it still wouldn't shock me either way.