What does the VPN provider claim when they receive a court summons for copyright infringement?

In the US at least, they are neither liable for the actions of their users nor under any sort of obligation to retain IP address logs. So, as far as I can tell, they're free to respond with "sorry, can't help you." IANAL, though.

The EU on the other hand has mandatory data retention, but it seems that this has been challenged be several countries, according to https://www.eff.org/issues/mandatory-data-retention

PIA (before acquisition) said in lawsuit that they can't provide IP address because they don't save.

They don't receive a summons, because the courts in their jurisdiction don't issue summons for that.