If you are a fast learner, then you can get some pdf or read books that would increase your understanding about what Bitcoin mixing really entails and the mechanism that the work done without any issue or problem.
If you want to use a mixer like Unijoin, then you do not have to go that far. It is not rocket science, and the mixing process is easy, even for a newbie, all they need is to create a new BTC address to receive mixed coins and set the delay time. Once you deposit the coins, the mixer will handle the rest.
It is entirely unnecessary for him to learn the technical operations of Bitcoin and, by extension, the Blockchain, just to enable him to use coin mixing services like Unijoin.
In my opinion, as long as you have been in the crypto space and have ever transferred and/or transacted Bitcoin or any other cryptocurrency for that matter, then you can easily use coin mixing services.
However, if he is suggesting to read up on Blockchain and Bitcoin, then he is probably inferring that coin mixing services like Unijoin are difficult to use and practically unsafe unless, of course, who he is replying to is interested in understanding the underlying technology the coin mixing service is built upon, and in that regard, it makes sense to refer him to a technical document not on Bitcoin but on the mixing service itself.
You are completely right and I 100 precent agree with you, it is just like being a driver, one does not have any need to understand how a car engine functions to drive the car, you just simply learn how to drive, which consists of turning the steering as it should, while shifting gears as it should (for manual drive), for automatic drive, one doesn't even have to shift gears or have to know anything on how the gearbox works, they just enter a car and drive.
Same exactly as you have explained it, using a mixing service only needs the user to know how to send and recieve crypto, that all, there is absolutely no need going into reading the technical aspect of how bitcoin functions or so, doing this may likely create confusion in the mind of the user, who may likely see it all as very difficult to understand, where as it's something that is very easy.