I’m surprised they volunteered that, but then these are pretty affordable as far as premium connectors go, these days. They’ll probably say the layer is so thin (e.g. a few atoms thick) that it doesn’t affect sound nor electrical resistance, and helps stabilize the metal interface. But then you’ll have other guys who claim nickel sounds awful in any amount. Nickel is not a good conductor, but it’s very stable.
AQ doesn’t use a nickel layer in their silver dipped or plated connectors, but then they tarnish like crazy.
Probably a good call to go for copper, but you'll want to keep them clean.