I have dealt with this situation in the past.
You have a non-governmental entity like a nonprofit Board of Directors (which is what HOA's are; a type of 501(C) organization). They have the legal right to craft rules within a certain purview, i.e. not conflicting with State or Federal law and not conflicting their organizational Bylaws that require a higher approval (e.g. member vote).
Now if you're in court trying to avoid HOA penalties for past actions, arguing the rule was ambiguous, that will be what it is. However...
For future actions, if it is within the purview of the HOA to ban the activity, then they can interpret their rules as having banned it.
Why? Because they could just as easily ban it explicitly with a simple rule change. "Six of one, half dozen of the other".
The common novice blunder is to try to gain an advantage by "mincing the words" of a Bylaw or other rulemaking, when that council has a right to change that rule at any time. Such efforts are misguided.
Because the council simply can change the rule.
Even if you take them to court. A savvy judge is going to recognize that the HOA or Board is the court of first impression for their own rules, especially rules which they have the legal right to change at any time.
The most the homeowner could hope for is the judge calling everyone into chambers and asking "When is the next HOA meeting? If the HOA really wanted to pass a rule explicitly banning non-government flags, could they do it at that meeting? Yes? Good. Then come back to me with recorded minutes of that meeting. I will rule then."
At which point, obviously, the judge will side with the HOA if they pass a ban, and not if they don't. In effect, the judge is kicking the issue back down to the "lower court". (where it belongs).