I had no trouble detecting the sarcasm A Thing wrote - because context matters. The horned helmet thing is widely known to be a myth, so his ", right?" on the end alluded to the misconception itself. Asking for validation for a well known false statement is a deliberate give-away that you're being sarcastic. Showing that a comparable linguistic construct contains no irony or sarcasm misses the point. Sarcasm and irony rely on saying the opposite of what you really mean.
e.g. calling Abe Lincoln "one of the greatest Republican presidents of all time" vs calling Trump "one of the greatest Republican presidents of all time". Linguistically, they are as identical as the horned helmet vs the ship-shields. But one is clearly sincere while the other is clearly sarcastic. Context makes all the difference for sarcasm, and showing that a near identical statement lacks sarcasm isn't sufficient to disprove sarcasm.
Well the use of “right?” implies a rhetorical question, meaning the statement prior to it is either obviously true or obviously false. Given I don’t know A Thing, I don’t know whether or not they are aware of the well-travelled myth (in which Vikings in media are generally shown/described with horned helms to show they are Vikings) so was unaware of which state they meant the rhetorical question.
Didn’t expect two paragraphs of textual analysis on how irony works
ah well. Irony doesn’t work well in a non-verbal medium, because there are no tonal or body language cues with which to discern whether or not the phrase is to be taken at face value or means the opposite of what was actually put down.