The words "valuation" and "overvalued" appear exactly 0 times in the post. The author didn't make the argument you're claiming.

Actually I misspoke in my original comment. NVIDIA has about $100 billion/year of free cash flow. So $25 billion is a quarter's free cash flow, not a half a year's.

$25 billion of debt against a $5+ trillion equity value and ~$100 billion of annual free cash flow barely moves the needle on equity beta or default risk.