Shaw's highborn characters regularly engage in hyperbolic banter that satirizes them as schoolyard bullies. In Scene 2, Robert's letter shows up in the midst of one of these bantering sessions between Charles and La Trémouille:
LA TRÉMOUILLE [handing over the paper to the Archbishop]. Here: read the accursed thing for me. He has sent the blood boiling into my head: I can't distinguish the letters.
CHARLES [coming back and peering round La Trémouille’s left shoulder]. I will read it for you if you like. I can read, you know.
LA TRÉMOUILLE [with intense contempt, not at all stung by the taunt]. Yes: reading is about all you are fit for. Can you make it out, Archbishop?
La Trémouille claims that the dauphin has so angered him that "he has sent the blood boiling into my head" so that he can't look at the letter and make out what it is. Obviously, this statement is hyperbolic. La Trémouille would be dead if his blood were literally boiling. And if he were really so angry as he makes himself out to be, he would hardly still be in the room strategizing with the dauphin. He is simply trying to get another shot in at Charles by claiming that Charles has ruined his eyesight.
Charles can't pass up the opportunity to tease La Trémouille. Anger so bad that he can't see straight sounds like a convenient cover for not being able to read. Shaw expects his audience to be laughing at this exchange and to be taken by surprise and delight when La Trémouille comes up with yet another comeback: the dauphin can read, but he can't do much else. An entire room full of men has to make all his political decisions for him. Once again, hyperbole is at play. Charles does need a lot of help to rule, so the insult cuts. But it remains funny and not cruel because Charles is not quite so pathetic as La Trémouille makes him out to be.
Although this banter is entertaining, it does not inspire a great deal of confidence in political leaders. Shaw makes these men out to be petty and overly secure in their powerful status. Charles doesn't see a problem with his over-reliance on advisors because he takes it as a given that he deserves to be dauphin and soon king. La Trémouille is clearly not worried that he might be kicked out of these political discussions. The men making huge decisions on the world stage do not have any qualms about behaving childishly. Shaw thus calls into question the respect they are owed.
In the Epilogue, Ladvenu tells Charles that Joan has been cleared of all charges. He uses hyperbole and a hint of verbal irony as he reassures Charles that he wasn't crowned by a heretic:
CHARLES. Good. Nobody can challenge my consecration now, can they?
LADVENU. Not Charlemagne nor King David himself was more sacredly crowned.
CHARLES [rising]. Excellent. Think of what that means to me!
Ladvenu compares Charles to two of the most legendary kings. Charlemagne was an emperor who united most of Western Europe for the first time since the Western Roman Empire had fallen to Byzantine rule; Charlemagne was the first Western Roman Emperor in three centuries, and he launched a massive campaign to spread Christianity throughout Europe. King David is an even more significant religious figure. In the Bible, he is most famous for his rags-to-riches ascension after he kills the giant Goliath, proving himself worthier to rule than the neglectful King Saul. Ladvenu is being hyperbolic in his comparison. It would be nearly impossible for anyone to be as "sacredly crowned" as these larger-than-life figures. By invoking them, he reassures Charles that his crown is as safe as safe can be.
But Shaw, if not Ladvenu himself, is being ironic as well as reassuring. Charles VII is not an especially legendary king—this much Shaw and his audience know. Most of his notable successes were in fact Joan of Arc's successes. On its surface, the statement that "Not Charlemagne nor King David himself was more sacredly crowned" means that Charles VII's crowning was as sacred as that of the most important kings in history and mythology. On a deeper level, the line means that none of these kings were sacredly crowned. Or, if they were, to be sacredly crowned does not mean much of anything.