My USAF DOD civilian experience is most DOD ATC positions are classified as "key". Before you can hire into a "key" position, you have to sign a "key" position letter. It basically asks you to state whether you have recall status (IRR, retired military, standby reservist, retired reservist, etc), and if you do, notifies you that they may remove you from that status if alternate staffing for your civilian position is unavailable.
If you do or don't have recall status, you agree not to join a reserve unit without approval of management and civilian personnel approval.
JS ATC, the answers that you got are all right on the money, and good information to know. What you need to do, is make sure that in your resume, or at interview, or additional information submitted states that you agree to a key position status, and will not deploy unless released to do so, or prior to joining a reserve unit. A lot of DoD places will use this as a reason not to hire you, because they either don't know the rules and how they are applied, or they erroneously use it as justification to get to someone else on the list that they already have in mind. Up to you to make sure that they know you are in it to win it and want the job.