As a developer, I completely understand this.
loop:
Manager: how long will this feature take?
SWE: By lunchtime, tops
3 weeks later:
Manager: feature complete, right?
SWE (sweating bullets): We're just testing the final bits
goto loop
loop:
Manager: how long will this feature take?
SWE: By lunchtime, tops
3 weeks later:
Manager: feature complete, right?
SWE (sweating bullets): We're just testing the final bits
goto loop
I'm not trying to claim that each individual claim of Musk's has this potential behind it, but equating it to day-to-day situations of underestimating the difficulty of a small task at your day-job oversimplifies the matter way too much and just makes excuses for someone who should TRULY know better.