You want to use an ID that can predictably be unique for something like this. You shouldn't use a random string.
As a way to generate unique ids this isn't horrible. 12^62 is something like 220 bits. The odds of a collision are even lower than with a UUID.
Guaranteed uniqueness is preferred, yes. But the level of effort needed to guarantee uniqueness across a large application / dataset / etc is much higher than "unique enough", just as it's a lot more expensive to prove a number is prime than to generate a number that is 99.9999% probably prime.