Somehow I don't think that modern Stoics would accept this argument if it were put the other way--if it were claimed, for example, that Torquatus, Lucretius, or Philodemus understood Stoicism better than any modern Stoic ever could.
While there's no question Epictetus, Cicero, Arrian, Plutarch, Paul the Apostle, et al. would have known more about Epicureanism than we do, would have known practicing Epicureans, and would have had access to Epicurean texts now lost to us, it's also no question that they all had axes to grind, were no friend to the philosophy, and had no reason to give an unbiased report. There is no reason why we shouldn't take a cautious approach to what they wrote about a rival philosophy while at the same time being grateful (??) to them for transmitting to us what they did. At the same time, these are the same people and their ilk that burned, threw away, and generally discarded the Epicurean texts in the first place, so my gratitude only goes so far.