Technically, it is. Bitcoin never had ICO and there were no premined sales. But there is a nuance. But what about the early miners and investors, now called whales, who mined, bought, or otherwise accumulated a lot of BTC? Unequal distribution among the owners of bitcoin still exists (ever since), which is not entirely correct from the point of view of decentralization.
You can't even begin to compare premine with early adopters because there is a difference between an unequal distribution and unfair distribution.
As a rule of thumb if you can find a single unit in a cryptocurrency that YOU couldn't mine, that cryptocurrency has an unfair distribution. But if you can find some coins that YOU
chose not to mine that is an unequal distribution not unfair.
That is why premine is unfair, the creator of that shitcoin is the only one who is capable of printing those coins making it unfair and centralized.
One being early or having invested more than others doesn't make it unfair, it just makes it unequal because there was no restrictions or preference being enforced on anybody preventing them from mining any of the bitcoins in circulation starting from block #1 and the very first block reward.
They even roll back and forked the coin. Ethereum isn't even in their original chain, Ethereum Classic is.
With Bitcoin, too, not everything is so simple. What about Bitcoin Cash? It's also a forked coin. At this point, there are similarities between ETH and BTC, right?
Again you are making a wrong comparison.
The fork that ethereum had was to roll back blocks, which is to effectively remove one of the main principles of cryptocurrencies called immutability of their blockchain. So ETC still is immutable while ETH is not.
On the other hand things like bcash are copies of bitcoin that have nothing to do with bitcoin. They just copied the code and the chain (history) to create a new coin.