I think Elon tweeted that because he still wants to prove what he said in 2018 can and will be done. -Probably. However, what at the time seemed like excessively flamboyant engineering proclamations by Elon on twitter very often turned out in the end to be deep and important engineering advancements. So this explicit, and incongruous, micrometer reference could be something more profound than just bragging rights. A for all practical purposes perfect fit and finish is at most a milimeter issue, not something 1000 times smaller.
"Our car needs to be designed and built with such accuracy and precision that, if an owner measures dimensions, panel gaps, and flushness, and their measurements don't match the Model 3 specs, it just means that their measuring tape is wrong."