Can we say this? A startup is more likely to fail because it couldn't iterate fast enough than because it didn't start with a powerful enough platform/stack?

If so, then you should pick the platform/stack that lets you iterate the fastest. Consider both your experience and your problem space when making the choice...

Is that really the lesson to take away? They have lots of projects that are not in PHP and designed a superset of the language with features it didn't have and their own VM for it, among other things, so, while they may have overcome the issues, I'm not sure that it didn't matter.