If you take a look at the diagram of Jambler then you'll clearly see. (It's classified as a Mixer)
2* Letter of Guarantee is issued by the Jambler themselves.
3* & 4* Jambler is responsible for sending and receiving the coins from the Customer, and Partner MIXER doesn't even hold the funds.
In 2018, Jambler created a full-cycle software and sells its software to those who want to have a mixing business. And if you type “mix bitcoins” into Google and somehow end up on their website, you will not be able to make a mix as an ordinary user. You will need to deploy an affiliate node, which is 99.9% likely to deploy only if you want to become a real partner of the jumbler.
Not to mention that Jambler did not change the concept of its business for 6 years (even after censorship was launched on January 1). And the letters of guarantee that you mentioned are an integral part of their automated processes, and are designed to provide users with reliable information that the software that partners use is valid.