Hey TeeTee, thanks for making this report.
I've talked the incident over with The Alchemist and we agreed that, since the punishment has been reviewed and as of now stands, the warn was not false, however he could have verbally warned you first so that you knew that there was a clash with the rules. Although you may not consider this prop shielding, it still involves shielding yourself with minecarts which gives you an unfair advantage.
Seeing that Alchemist now knows that in future he should verbally warn people when they don't know they are breaking the rules, you can consider this report accepted. However, he also claimed that you were shouting and/or disrespecting him after the event, something that is also punishable. In future don't lash out if you feel you have been mistreated or falsely warned, and make another forum report to avoid further punishments.
I would recommend that you read over the rules here if you haven't already in order to avoid further issues in the future.
Have a good evening.