The article says the parents limit this high-risk stock picking to 10-15% of their portfolio.

Edit: Excerpt:

>> My mom assured me that she’s started reading up on stock valuation, and only buys 20 or 30 shares at a time. She gauges that 15% to 20% of her wealth is in day trading, leaving the bulk of it in her tried-and-true index and retirement funds.

>>My dad, having gotten into the game later, has only about 10% of his money in short-term trades. In the unlikely case that he loses it all, which he doubts would happen, it wouldn’t be financially devastating.

Edit 2: And yes, in fairness, since they're just "gauging" it and might not keep rigorous track of the actual values, it could be a huge underestimate. But still, not "goodbye inheritance".