The question is: if you had not looked at chardet's source would you still be able to create your work? If the answer is 'yes' then you probably shouldn't have looked at the source, you just made your defense immeasurably harder. And if the answer is 'no' then you probably should have just used chardet and respected its license.
Copyright is automatic for a reason, the simple act of creation is technically enough to establish copyright. But that mechanism means that if your claimed creation has an uncanny resemblance to an earlier, published creation or an unpublished earlier creation that you had access to that you are going to be in trouble when the real copyright holder is coming to call.
In short: just don't. Write your own stuff if you plan on passing it off as your own.
The accuser just needs to establish precedence.
So if you by your lonesome have never listened to the radio and tomorrow morning wake up and 'Billy Jean' springs from your brain you're going to get sued, even if the MJ estate won't be able to prove how you did it.
So my (perhaps naive) understanding is if none can be found, then the author of chardet 1-6 simply doesn't have a case here, and we don't get to the point of asking "have you been exposed to the code?".
The similarity is an established fact - the authors claim that this is chardet, to the extent that they are even using the chardet name!
Had they written a similar tool with a different name, and placed it in its own repo, we might be having a very different discussion.
You'd have to make that claim absent any proof and then there better not be any gross similarities between the two bodies of code that can not be explained away by coincidence.
And then there is such a thing as discovery. I've been party to a case like this and won because of some silly little details (mostly: identical typos) and another that was just a couple of lines of identical JavaScript (with all of the variable names changed). Copyright cases against large entities are much harder to win because they have deeper pockets but against smaller parties that are clearly infringing it is much easier.
When you're talking about documented protocols or interface specifications then it is a different thing, those have various exceptions and those vary from one jurisdiction to another.
What can help bolster the case for the defense is for instance accurate record keeping, who contributed what parts, sworn depositions by those individuals that they have come up with these parts by their lonesome, a delivery pace matching that which you would expect from that particular employee without any suspicious outliers in terms of amount of code dropped per interval and so on. Code copied from online sources being properly annotated with a reference to the source also helps, because if you don't do that then it's going to look like you have no problem putting your own copyright on someone else's code.
If it is real, then it is fairly easy to document that it is real. If it is not, after discovery has run its course it is usually fairly easy to prove that it is not if it is not.
By contrast, in Germany (IIRC), false confessions are _illegal_, meaning that objective evidence is required.
Many legal systems follow the principle of "innocent until proven guilty", but also have many "escape hatches" that let them side-step the actual process that is supposed to guarantee that ideal principle.
EDIT: And that is just modern society. Past societies have had trial by ordeal and trial by combat, neither of which has anything to do with proof and evidence. Many such archaic proof procedures survive in modern legal systems, in a modernized and bureaucratized way. In some sense, modern trials are a test of who has the more expensive attorney (as opposed to who has a more skilled champion or combatant).
If you wish to be able to claim in court that it is a "clean room" implementation, yes.
Clean room implementations are specifically where a company firewalls the implementing team off from any knowledge of the original implementation, in order to be able to swear in court that their implementation does not make any use of the original code (which they are in such a case likely not licensed to use).