Now the question is this 40M Euro belong to Chipmixer?
No. It is likely that the funds do not rightfully belong to ChipMixer from a legal standpoint, but instead to their clients. (Not sure if the term "legal" is appropriate in this particular context.)
I don't think that people who mix their coins on chipmixer, their money is stored there because the money is moved to the user's wallet as soon as the mixing is done.
Actually, that's not quite right. When you use ChipMixer to mix your coins, you receive "chips" after depositing your coins. These chips are essentially pre-funded addresses owned by ChipMixer, and you're given the private keys to withdraw the funds. However, not everyone immediately transferred the funds from these chips to their own wallets, which resulted in the funds being confiscated.
So where does this money comes from and why it is stored at chipmixer?
I presume that these are the unutilized chips that were available on the platform for the company's operational requirements, as well as the chips belonging to customers who did not withdraw them to their external wallets.