Does it target the same customers. Yes
We want something to put in our pockets that can browse the net and send email.
Do they both do that. Yes.

It's a competitor. OS is irrelevant.

The MAC is a competitor to the PC. A Unix server will compete with a Windows server.

What will make the difference is the individual features that the purchaser wants. For example whether he wants windows or Unix. Five hard disk or 2. SATA or SCSI. USB portsd or legacy Serial and parallel ports.

But the servers in their entirety, to function as servers are competitors.