What does " the sophistication of the defendant" mean?

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WikiLeaks founder Julian Assange has been charged under seal, prosecutors inadvertently revealed in a recently unsealed court filing — a development that could significantly advance the probe into Russian interference in the 2016 election and have major implications for those who publish government secrets.

The disclosure came in a filing in a case unrelated to Assange. Assistant U.S. Attorney Kellen S. Dwyer, urging a judge to keep the matter sealed, wrote that “due to the sophistication of the defendant and the publicity surrounding the case, no other procedure is likely to keep confidential the fact that Assange has been charged.” Later, Dwyer wrote the charges would “need to remain sealed until Assange is arrested.”

Dwyer is also assigned to the WikiLeaks case. People familiar with the matter said what Dwyer was disclosing was true, but unintentional.