My reasons are laid in the link to the previous post at the beginning of the article.

• Lower rates
• Don’t need to liquidate investments (and incur capital gains) to pay down payment
• Foreigners aren’t eligible for mortgages anyway

IB follows the law on margin, so it depends on what country you are in. If you are American, they will do either Regulation T or Portfolio Margin depending on which one you sign up for and quality for. Regulation T is limited to 50% of the portfolio. Portfolio Margin is a more complicated calculation based on the exact contents of the portfolio and how risky they are. Assuming you hold a diversified portfolio of index funds you borrow more like ~1x of your portfolio.