SimplyJetpacks has many "similarities" with IC2's implementation both on the macro and micro level. It starts with having the same overall approach, continues with the class hierarchy, solutions to individual problems, partially identical method and variable naming, semantics, down to exactly matching syntactical elements at times. While it may be a coincidence that some of these happen to end up the same if someone implements a similar thing himself, the probability decreases with each instance and there are a lot of those. Additionally there are some occurrences where he's using implementations seen in IC2 which are not reasonable for the scale of his mod or which are just legacy from the past due to IC2 just being a mod much older than e.g. Forge. There's also fairly obvious implementation style/approach inconsistencies towards the features he has added of his own.
Overall it'd be very naive to claim that SimplyJetpacks did not derive from IC2's implementation. The mod authors disagreeing are simply not considering the combined likeliness of all factors or are biases themselves. If law required absolutely bulletproof evidence, there would be no proof, which isn't the case. I have no doubt that there is very strong combined evidence to prove that SimplyJetpacks does indeed use code taken from IC2, it'd be up to Tonius to prove the opposite from that point.
I'm not interested in suing anybody, esp. over a mod, but if he doesn't comply I have no options besides either doing so or tolerating it. Considering how easy it'd be to just make it an IC2 addon which can be made to work with thermal expansion as well or just implementing it from scratch, I have to assume malicious intend and thus can't tolerate it. It is not enough to ask to at least provide a non-derivative implementation if you copy the idea already.
I did ask him discretely to take it down, but if that doesn't work I'll have to enforce it. If we didn't care, the mod would be public domain. Licenses and copyright are worthless if you don't back then up in the end.
It's also quite the antisocial move from some people to push Tonius into non-compliance, it's mostly his risk to take, not theirs.