I think what needs to be better understood here, and what I don't think has been clearly stated, is what warranty the OP got when they purchased the car used from Tesla.
The current used vehicle warranty is:
1 yr, 10,000 miles additional coverage beyond the original 4 yr, 50k mile warranty.
or for car where the original 4 yr/50k mile warranty has expired, then 1 yr/10k miles from date of purchase.
I do not recall when this change in used warranty terms occurred, before or after OP purchased their car used. Used purchases originally had even more warranty attractive additional warranty than now. Given they imply the original 4 yr/50k is still active, then I would suspect they have an additional 1 yr/10k miles warranty after the 4y/50k ends per the current terms.
Now, if they wanted to try and negotiate with Tesla to give up the added 1 yr/10k miles of used car full warranty coverage in order to purchase the ESA, at an added cost and with it's $200 per service visit deductible, I'm not sure what Tesla would say. But you'd actually be paying to have worse coverage for the first 1 yr/10k miles after your original 4yr/50k expired.
It's this interaction with the used car warranty, which extends beyond the original 4yr/50k warranty which I think is behind the statement that you cannot buy the ESA on a used car purchased from Tesla, versus you purchased the same car from a private party but without the added used car warranty.
For reference, here's the current used car warranty document from Tesla website.
