Your confusion comes from the assuption that every mixer works like CoinJoin; whereas ChipMixer is basically an off-chain mix.
Even before ChipMixer, "coinjoin" was never the standard: a user would simply get someone else's coins in return for their own. Coinjoin is the only form of mixing that leaves an on-chain trail to follow.
True; CoinJoin is actually newer than I thought. According to
Bitcoin Wiki CoinJoin page, the first mention of the idea of CoinJoins was in 2013, as quoted below.
Ever since I was a wee lad I've had a dream .... a dream of being incorrectly assessed as impossibly rich by brain-dead automated analysis. Now with your help I can be!
Here is how it works: A lot of people mistakenly assume that when a transaction spends from multiple addresses all those addresses are owned by the same party. This is commonly the case, but it doesn't have to be so: people can cooperate to author a transaction in a secure and trustless manner. We can make it a lot easier for people making this mistake to discover their folly by making there be a single address that seems linked to everything.
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I remember in the past mixing was actually more commonly referred to as 'tumbling'; maybe indeed better fitting for something where you throw in coins and get completely unrelated coins of equal value back out. Instead of a CoinJoin mechanism that semantically appears more as a 'mix' since you throw together your inputs with other people's UTXOs, mixing them and getting something out from that same pool.