Could afford it rather easily when one takes a few minutes to check the numbers. Salary in 2003 was $154,700 based on public records adjusted for inflation that is $275,820 today. A pre-tax monthly income of $12,981 or $22,985 adjusted for inflation.
Home value of $870,000 - 20% for down payment = $696,000 after modern fees. That puts monthly payment at $4,626. That would be roughly 20% of pre-tax income. Substantially below what banks preferred (28%).
Add to the fact he was an Attorney for at-least 8 years before becoming a State Senator, then purchased the home while being a House representative banks likely felt his low risk profile already coupled with his career and forward facing position made it a good loan.
There no conspiracy here. The loan amount, salary, and percentage of income very easily affords that home.