Is it true that all entrepreneurs are small business owners?

No!

Entrepreneurs come in all sizes and shapes from Architect employees to Zebra herders (might as well have some humor).

Entrepreneurs have a definition that is related to their wealth and success not how they got it. Though their are some rules to how they got it (you can't rob a bank) but other than that small rule you probably pass hundreds of them in your everyday life. (they aren't as exclusive as your TV says)