There are a few varieties of convertible debt -- one is with a discount, and another is with a pre-money cap.
Lots of investors will insist on pre-money cap at a reasonable seed valuation. This basically means later when you do a series A or change of control, the investor's money converts with the Series A, and they get reimbursed typically with additional common stock that would simulate that valuation as if they did equity.
Straight up convertible debt with only a discount actually does NOT align the entrepreneur's goals with the investor... the investor gets a fixed percentage discount off the Series A price, but what if they put in $100k at year 0 and the company is now worth $10 million later at year 1 (rare but not unrealistic example)? That $20k profit with the 20% discount is starting to look like an awful move.
History lesson: Back in the day, notes were common just to really "bridge" startups from seed to Series A. They were issued to startups by the same VC that was already planning to do a Series A with them -- so the discount was fine because the VC would set the valuation as well. It's different now because your note investor (angel, friends or family) probably won't be the pricing your next round -- more likely, the market will.
Now it just doesn't make sense for most investors since they can take a chance on a team, do a note with a discount, and then see no significant return if the team is successful and raises the next round at a much higher valuation.
This article is actually kind of messed up because it assumes that you priced your round poorly, and then uses that as an explanation for why debt is good. If everyone is doing their job, and you're planning for success, then do a cap... or heck, do equity with YC's Series AA.