I'm pretty sure that the terms of Getty Images are that they won't even let the photographers use their own photos. When a photographer signs an image over to Getty, they assign Getty all rights to licence it. It's exclusive to Getty and their customers. So the photographer had lost control of that one. He cannot give permission, and he cannot use it himself. This is one reason why I don't sell through Getty.
Royalty free doesn't mean you don't have to pay for a photo. It means if you pay for a licence to use the image, you can keep it on your computer and don't have to buy a new licence for each new use - but you still need a licence to use it in the first place.
If the photographer had signed over licencing to Getty and you didn't buy a licence, it was a breach of copyright, which is why they were within their rights to follow it up. And they're well known for doing that. You were unlucky too because if it had been owned and managed by anyone else they might have been more sympathetic.
We should never assume that 'royalty free' means free. It doesn't. It means you don't have to pay royalties to the photographer for every single use. You still need to buy a licence to use it the first time.
This is a great story though. I've been had by Getty Images in the past too. A work colleague did what you did, and I got a £1000 fine within weeks of starting a new job because of this previous employees error. My company paid it. https://medium.com/life-tips/i-got-a-1000-demand-for-using-an-image-off-the-internet-8c7d236126cf
Would you mind me using your story as an example of why people should always use creative commons, public domain, buy a licence, or take their own pics, because royalty free doesn't mean free? In a Medium article, I mean?