The article doesn’t say when Facebook used Instagram’s photos. There was a time when Instagram was not integrated into Facebook’s infrastructure. I don’t know

The article doesn’t say when Facebook used Instagram’s photos. There was a time when Instagram was not integrated into Facebook’s infrastructure. I don’t know how deeply Instagram is integrated today but they appear to at least use Facebook’s storage and CDN. So if this research(?) was done lately, “fed […] into its own system” could refer to Facebook’s spam detection system as opposed to Instagram’s spam detection system.