Dedicated plans are not artificially limited in any way - what does that mean?

Dedicated plans are not artificially limited in any way. The maximum performance for a plan is determined by the underlying instance type and the plan size (the server hardware).

We don't have any limits on the number of vhost, users queues etc. But e.g. too many queues (thousands and thousands of queues)  might use to much RAM and kill the server due to OOM.