Even if the hoplites don't report the attack, you'll know something is up when you lose radio contact.So you are sure these panzers won't come from the right, because your hoplites will stay alive long enough to even tell you what happened? ;x
It's just not only unbalanced to help flanking with outdated units, it's just unrealistc too ^^
Strength should matter somehow.
And if a tank blitzes through the hoplites and attacks your modern unit in the same turn, well, hey, your modern unit lost the flanking bonus. Works as intended.