Hi Paul,
IMO there’s no. of ways to find B2B leads contact info on the internet (when it comes down to cold emailing). Tools like apollo.io, rocketreach, etc. helps to find professional email ids and other business details. I assume, that’s how they’ve contacted your client.
To answer your second question - it’s easy to identify what ESP any brand is using (in this context Klaviyo); it’s simply by signing up on the prospect website or subscribing to newsletters and then looking into email headers.
However, I agree with @chloe.strange , since they are not associated with Klaviyo, they might have a system or algorithm to do that… You can probably contact them to understand more about their product.
Hope that make sense. Let me know if you have any questions further.
Thanks,
Mohsin