I read the following announcement with great interest.
https://aws.amazon.com/about-aws/whats-new/2018/11/announcing-amazon-dynamodb-on-demand/
The new "on-demand" feature really helps with capacity planning. Reading the documentation, I can't really see if they do some "magic" to resolve the problem of hot partitions, and partition key distribution.
Is partition key design just as important if you provision a table "on-demand"?
Yes, partition key design is just as important. That aspect has not changed.
Since you mentioned adaptive capacity in a comment, one thing to make sure is clear. Once it is on for a table, it is on and DynamoDB is monitoring your table.