The tax code has it's own definition of "disabled" with respect to the early-distribution penalty exception which is separate from the definition used by the Social Security Administration. To qualify for the penalty exception you must meet the tax-code definition and IRS requirements provided in IRS Pub 590-B:
You are considered disabled if you can furnish proof that you can't do any substantial gainful activity because of your physical or mental condition. A physician must determine that your condition can be expected to result in death or to be of long, continued, and indefinite duration.
It's entirely possible that the same documentation that you provided to the SSA would be sufficient to substantiate to the IRS that you are eligible for the disability exception to the early-distribution penalty, but your Social Security disability determination alone is not sufficient.
With proper documentation to support eligibility for the exception to the early-distribution penalty, yes, you can amend your 2018 tax return to claim this exception (Exception number 03) on line 2 of Form 5329.
