True.  

https://readiness.k2.com/download/movingappstok2cloud-doc

K2 CUSTOMIZATIONS
The following customizations are not currently available within K2 Cloud:
• Custom K2 SmartForm Controls
• Custom K2 SmartForm Themes
• Custom K2 SmartObject Service Brokers
• Custom User Managers
• Custom Inline Functions
• Custom code

It is correct that you cannout publish custom controls as you would today in K2 5, however if you think a little laterrally you can do some amazing things.

1) Save the required js,css ect hosted in the web (perhaps azure)

2) Drop a data label onto the form with a specific naming convention

3) Have a common form rule that knows to go and reference and load the scripts from the web hosted env and bingo you can have a custom control.

Now it will take more effort than this at first but I put the challange to you!