"I mentioned this as a theoretical possibility, and I am not saying that this is what is actually going on."
No, you offered not a possibility but a possible explanation. "The other possible explanation for the insider buying is that bad news of some sort is imminent. Let’s hope it isn’t that."
The difference is this. That a possible sequences of events is not illegal is not an explanation, to my way of thinking. If you tell me that he had flushed $10,000 down the toilet, the fact that that is not illegal is not an explanation of the deed.
If his purpose was to artificially support the stock price for some purpose that is not apparent to me, that sounds like market manipulation, which would be illegal.
I see no lawful explanation consistent with his purchase if he knows the results are going to be bad.