What they probably mentioned about 18X aka SF is when you go to MEPS as prior service, your grade determination waivers will go into consideration when HRC and ROC determine which MOS's you are available for. I had a grade determination waiver last winter and only MOS's that came back for E-5 was photographer, correspondent and 18X. Managers for 18X denied me because they wouldn't give a a 14 day DLAB waiver because I had to reattend AIT first.
I ultimately had me grade determination waiver changed to E-4 so I could come back to Infantry (Prior-service Marine for 12 years and seperated).
Honestly you being Navy, you are really behind the powercurve when it comes to Infantry. Start cracking on the run, proper Army pushups, and full old school situps. Start studying the Soldiers Creed, Infantryman Creed and song, general orders (different than Navy) common courtesies (different than Navy). Google 670-1 and FM 3-21.8 (Infantry Bible).
Words of advice, starting getting all of your correspondence course and college transcripts, deployment books, DD-214 award messages etc togethor. You'll need your DD-214 for your uniforms at AIT (Commandos out in town in Fort Benning only store carrying all USN/USMC ribbons). Once your check into your unit, you'll need everything to get your ERB updated.
The above information goes for any prior service soldier coming back to service. I been dealing with Admin for months trying to get my ERB updated. P.M. me if you have any more questions.
Semper Fi and Hooah!