I'd argue this is one of the okay uses for petroleum as it's converted directly into a high value product and not emitted as pollution. I assume people don't do a lot of throwing away of Lego bricks. Going into the future I predict petroleum will be used to make material products a lot longer than it's used to make fuel. Even after we stop mining petroleum we'll still be turning farm crops into the same material.
You can make the same material by originating it from crops and it still wouldn't break down. Also the historical alternatives for this material in most applications are aluminum and glass which also will not break down in landfills.
I appreciate no offset shenanigans but did they factor in the fact that their product is made out of petroleum?