How do we know Antony is not really a "plain blunt man"?

I am no orator, as Brutus is;
But, as you know me all, a plain blunt man,
That love my friend, and that they know full well
That gave me public leave to speak of him.
For I have neither wit, nor words, nor worth,
Action, nor utterance, nor the power of speech,
To stir men's blood. I only speak right on;
I tell you that which you yourselves do know;
Show you sweet Caesar's wounds, poor poor dumb mouths,
And bid them speak for me. But were I Brutus,
And Brutus Antony, there were an Antony
Would ruffle up your spirits and put a tongue
In every wound of Caesar that should move
The stones of Rome to rise and mutiny.
It is noteworthy that Shakespeare has his Antony tell the plebeians that he is no orator but only a plain blunt man speaking extemporaneously--and then end the passage with a dazzling subjunctive sentence containing four striking images. Antony figuratively becomes Brutus and Brutus becomes Antony. Note the use of the subjunctive in “But were I Brutus” and in “…that should move the stones of Rome.” The mob experiences this oratorical magic and imagines that Antony, Brutus, Caesar, and the stones of Rome are all unanimously inciting them to riot.
Later, when Antony, Octavius, Brutus and Cassius parley on the battlefield at Philippi, Cassius will tell Antony:
The posture of your blows are yet unknown;
But for your words, they rob the Hybla bees,
And leave them honeyless.
No one in Shakespeare's audience would have expected such inspiration from Mark Antony, although they might have expected such fervor. Antony shows his brilliance in the final lines of his speech. Everything he says here is subjunctive—that is, impossible and unrealistic. It is pure poetry. Shakespeare knew that Antony could not suddenly become so inspired and eloquent without any prior indication that he had it in him. (It would sound like Shakespeare, not Antony.) Perhaps that is why Shakespeare gives Antony a soliloquy at the end of Act 3, Scene 1, to show that this "plain blunt man" does indeed possess the intellectual and imaginative potential to make such a moving and historically important funeral address.
O, pardon me, thou bleeding piece of earth,
That I am meek and gentle with these butchers!
Thou art the ruins of the noblest man
That ever lived in the tide of times.
Woe to the hand that shed this costly blood!
Over thy wounds now do I prophesy
Which like dumb mouths do ope their ruby lips
To beg the voice and utterance of my tongue,
A curse shall light upon the limbs of men;
Domestic fury and fierce civil strife
Shall cumber all the parts of Italy;
Blood and destruction shall be so in use,
And dreadful objects so familiar,
That mothers shall but smile when they behold
Their infants quarter'd with the hands of war;
All pity choked with custom of fell deeds,
And Caesar's spirit ranging for revenge,
With Ate by his side come hot from hell,
Shall in these confines with a monarch's voice
Cry “Havoc!” and let slip the dogs of war,
That this foul deed shall smell above the earth
With carrion men, groaning for burial.