Strange that this mixer doesn't provide letter of guarantee BEFORE i'll send coins.
A Letter of Guarantee can only be used if the mixer transfers mixed funds to a certain address. ChipMixer gives you private keys, and it's not possible to announce them before you deposit your funds.
If means in case of selective-scam (large deposits) nobody will able to prove that address really regarded to chipmixer.
Correct. That's why they're highly trusted here.
PS
Exchanges or casinos don't provide you with a signed message before depositing either.