I read the policy and Q&A at the link you provided... twice... but I stil can’t find the part that prohibits tattoos of historical American flags on one’s shoulder.
Additional information Google is your friend
B. THE FOLLOWING IS THE EXISTING POLICY PROMULGATED IN REFS A AND B.
(1) PRIOR TO ENLISTMENT INTO THE MARINE CORPS, APPLICANTS WITH MORE THAN FOUR (4) TATTOOS/BRANDS WILL BE ADMINISTRATIVELY REVIEWED IAW WITH REF B.
(2) TATTOOS/BRANDS THAT ARE SEXIST (EXPRESS NUDITY), RACIST, ECCENTRIC OR OFFENSIVE IN NATURE, EXPRESS AN ASSOCIATION WITH CONDUCT OR SUBSTANCES PROHIBITED BY THE MARINE CORPS DRUG POLICY AND THE UNIFORM CODE OF MILITARY JUSTICE (UCMJ), TO INCLUDE TATTOOS ASSOCIATED WITH ILLEGAL DRUGS, DRUG USAGE, OR PARAPHERNALIA, ARE PROHIBITED.
(3) TATTOOS/BRANDS THAT DEPICT VULGAR OR ANTI-AMERICAN CONTENT, BRING POSSIBLE DISCREDIT TO THE MARINE CORPS, OR ASSOCIATE THE APPLICANT/MARINE WITH ANY EXTREMIST GROUP OR ORGANIZATION ARE PROHIBITED.
(4) TATTOOS/BRANDS ON THE HEAD AND NECK ARE PROHIBITED.
(5) SLEEVE TATTOOS AS DEFINED IN PARAGRAGH 5A ARE PROHIBITED. HALF-SLEEVE OR QUARTER-SLEEVE TATTOOS THAT ARE VISIBLE TO THE EYE WHEN WEARING STANDARD PHYSICAL TRAINING (PT) GEAR ARE LIKEWISE PROHIBITED. NO ADDITIONAL ACTION IS REQUIRED BY MARINES WHOSE SLEEVE TATTOOS WERE PREVIOUSLY GRANDFATHERED PER MARADMIN 198/07.
4. THE FOLLOWING ADDITIONAL GUIDANCE CLARIFIES THE MARINE CORPS TATTOO POLICY AND IS EFFECTIVE IMMEDIATELY. MARINES WHO DO NOT MEET THIS GUIDANCE WILL BE GRANDFATHERED PER PARAGRAPH 6.
A. THE FOLLOWING APPLIES TO ALL MARINES. (1) TATTOOS/BRANDS ON THE HANDS, FINGERS, AND WRISTS ARE PROHIBITED.
(2) TATTOOS/BRANDS IN THE MOUTH ARE PROHIBITED.
(3) INDIVIDUAL TATTOOS VISIBLE IN THE STANDARD PT UNIFORM (SHORTS AND T-SHIRT) WILL BE NO LARGER THAN THE WEARER'S HAND WITH FINGERS EXTENDED AND JOINED WITH THE THUMB TOUCHING THE BASE OF THE INDEX FINGER.
(4) BAND TATTOOS NOT IN COMPLIANCE WITH PARAGRAPH 5B ARE PROHIBITED.
(5) TATTOOS/BRANDS WILL NOT BE VISIBLE OR APPARENT ON THE FEET OR LEGS WHEN WEARING THE SERVICE A, BLUE DRESS A/B, BLUE-WHITE DRESS A/B, OR THE EVENING DRESS UNIFORMS. EXCEPTION: FORMAL INSPECTIONS AS PRESCRIBED BY COMMANDERS AND/OR INSPECTOR GENERAL TEAMS.
(6) TATTOOS NOT VISIBLE OR APPARENT WITHOUT THE USE OF ULTRA-VIOLET LIGHTING (BLACK LIGHT) MUST MEET THE POLICY AS SET FORTH HEREIN AND IAW THE REFERENCES.
B. ADDED GUIDANCE APPLICABLE ONLY TO OFFICERS (COMMISSIONED/WARRANT).
(1) OFFICERS ARE LIMITED TO NOT MORE THAN FOUR (4) TATTOOS/BRANDS VISIBLE IN THE STANDARD PT UNIFORM.
(2) BAND TATTOOS EXCEEDING A MAXIMUM WIDTH OF TWO (2) INCHES ARE PROHIBITED.
(3) ENLISTED MARINES WITH PREVIOUSLY GRANDFATHERED SLEEVE TATTOOS ARE NOT ELIGIBLE FOR A MARINE CORPS COMMISSIONING OR WARRANT OFFICER PROGRAM. IN OTHER WORDS, AN ENLISTED MARINE GRANDFATHERED FOR A SLEEVE TATTOO IAW MARADMIN 198/07 HAS NO RESTRICTIONS FOR REENLISTMENT OR PROMOTION AS AN ENLISTED MARINE, BUT IS NOT ELIGIBLE TO APPLY FOR THE MARINE ENLISTED COMMISSIONING EDUCATION PROGRAM (MECEP), WARRANT OFFICER, OR ANY OTHER ENLISTED-TO-OFFICER PROGRAM.
(4) PRIOR TO COMMISSIONING/WARRANT OFFICER APPOINTMENT, OFFICER CANDIDATES WILL BE RESCREENED FOR TATTOOS WHICH ARE NOT IN COMPLIANCE WITH THIS POLICY.
Bottom line - Wait until you get through Boot Camp before you get a tat. One could argue that the flag in question falls into Section A3 - as the Confederate flag is used by White Supremesist groups. Whether he subscribes to their philosophy or not, the Confederate flag is NOT the US Flag. Basically, like all Regulations on the books - if they want to get you, they are gonna get you.