Bonds aren't binary from the perspective of an investor, they are priced and traded in the market just like equity. They can indeed do "okay". The market price reflects the coupon, timing to maturity and likelihood of default amongst other things. But yes, some of the metrics around Tesla point to a high level of risk. I doubt it was ever going to be any other way.
Point is, these bonds are probably not going to do OK....and believe me, the guy who wrote that understands bonds very well...in fact, he contends that in a normal market (not this bizarro one central banks have generated in the past 10+ years) if more investors understood bonds they wouldn't waste their time on stocks. If Tesla starts to tank, those bond prices will not go up - there is no way that level of risk should be priced at 5.3%. And the bond holder can wipe his behind with the paper the bond is printed on. Something rated as junk should yield better than these...but it's the cachet of Tesla.
Of course, there is no way many of the countries in the EU should have bonds yielding as low as they do for the risk they represent - countries with debt at 136% of GDP and yielding around 2% on a 10 year note? Preposterous.
Elon Musk is a very innovative, smart, and talented guy, but I maintain his greatest skill/talent is extracting money from people (and the taxpayers) to fund his schemes even though he loses money. He's pulled about every trick in the book so far, and especially as Big Auto gets into the market and price pressure becomes a reality, I think Tesla will suffer. A lot.