The security interest is equivalent to heilach, tendering payment of the debt, because it empowers the court to seize and sell the defendant’s land to cover the debt.
If the creditor responds to this unsolicited admission by thanking the debtor for reminding him of the debt, the finder is still absolved from taking an oath.
The reason one witness supporting the defendant excuses him from taking the Modeh Bemiktzat oath of partial admission is based on the same logic that obliges him to take the oath when one witness testifies against him.
Even if the litigant is sophisticated in matters of Jewish business law, he still must specify his pleadings.