I wonder if the person you talked to actually knew what they were talking about. Swappa explains what an unlocked device means in their FAQ and a bootloader is not mentioned.
That's sad, because one of Swappa's main selling points (and the reason they got popular) is that they started out as a marketplace for Android phones, and should specifically know about rooted / bootloader unlocked / etc. phones. Enthusiast stuff.
Their About page says:
> The inspiration for Swappa sparked when Ben had trouble finding a good source for test devices for Android development projects.
I guarantee you that most people buying used phones are more concerned if the phone will work with their existing phone plan than if they can use it as a development testbench.
https://swappa.com/faq/answer/unlocked-device