To help lend some clarification here, currently in the workflows you can send to a number or manually select a user (see screenshots). This becomes a manually and repetitive task as the user will need to create a separate if/then branch or workflow in order to send an SMS to a record owner.
If we look at the send internal email action, we can see the option to send to a contact owner is available; allowing the user to easily create and execute the following actions once and allow the Workflow to send to the designated owner automatically. This type of functionality would ideal with SMS.
Current process to send an SMS to an owner
Send Internal Email Functionality
Send Internal SMS Functionality