The conclusion that you have drawn is based on a misinterpretation of what constitutes as "capital". If "capital" meant "government debt", then the rate of return on capital would indeed equal the interest rate on government debt. However, when economists refer to capital, they are referring to the market value of the cumulative stock of a nation's capital.
What is "capital"? Piketty defines capital as:
"the sum total of nonfinancial assets (land, dwellings, commercial inventory, other buildings, machinery, infrastructure, patents and other directly-owned professional assets) and financial assets (bank accounts, mutual funds, bonds, stocks, financial investments of all kinds, insurance policies, pension funds, etc.) less the total amount of financial liabilities (debt)."
Furthermore, national capital can be broken down into public capital and private capital:
"[National capital is] the sum of public capital and private capital. Public capital is the difference between assets and liabilities of the state (including all public agencies), and private capital is of course the difference between the assets and liabilities of private individuals."
...and can be broken down into domestic capital and net foreign capital:
"Domestic capital is the value of the domestic capital stock (buildings, firms, etc.) located within the borders of the country in question. Net foreign capital...is the difference between assets owned by the country's citizens in the rest of the world and assets of the country owned by citizens of other countries."
The rate of return on capital:
"...measures the yield on capital over the course of the year regardless of it's legal forms (profits, rents, dividends, interest, royalties, capital gains, etc.), expressed as a percentage of the value of capital invested. It is therefore a broader notion than the 'rate of profit', and much broader than the 'rate of interest', while incorporating both."
What is "capital"? Piketty defines capital as:
"the sum total of nonfinancial assets (land, dwellings, commercial inventory, other buildings, machinery, infrastructure, patents and other directly-owned professional assets) and financial assets (bank accounts, mutual funds, bonds, stocks, financial investments of all kinds, insurance policies, pension funds, etc.) less the total amount of financial liabilities (debt)."
Furthermore, national capital can be broken down into public capital and private capital:
"[National capital is] the sum of public capital and private capital. Public capital is the difference between assets and liabilities of the state (including all public agencies), and private capital is of course the difference between the assets and liabilities of private individuals."
...and can be broken down into domestic capital and net foreign capital:
"Domestic capital is the value of the domestic capital stock (buildings, firms, etc.) located within the borders of the country in question. Net foreign capital...is the difference between assets owned by the country's citizens in the rest of the world and assets of the country owned by citizens of other countries."
The rate of return on capital:
"...measures the yield on capital over the course of the year regardless of it's legal forms (profits, rents, dividends, interest, royalties, capital gains, etc.), expressed as a percentage of the value of capital invested. It is therefore a broader notion than the 'rate of profit', and much broader than the 'rate of interest', while incorporating both."