Two ways:
1. As a receiver, you can emit the nostr event that means "zap received" without having necessarily received LN sats. You can create the invoice, use the preimage, and fake the contents of the zap note.
2. You can also use sockpuppet accounts to zap yourself. You could recycle lets say 100 sats over and over (send zap, receive zap, send sats back silently via LN, repeat). You can thus inflate the amount of zaps recived as high as you want, without having received that actual amount.
So, aren’t zaps possible using pure lightning?
No, both sender and receiver have to support zaps.
A pure LN wallet cannot send or receive zaps.
You mean the sending nostr client needs and the receiver ln wallet need to support zaps
For sending: AFAIK you cannot zap from a normal LN wallet (you need a nostr component to initiate the zap)
For receiving: yes, you cannot receive zaps on a normal LN Address (or LNURL endpoint). The LN Address has to be customized with some more fields + has to monitor the pending invoices / zaps. It also needs a nostr component to emit nostr events.
Btw I think sooner or later they will be possible via pure LN (+ send optional proof via nostr).
It's not possible now because when paying, the preimage (proof of payment) is only available in the LN wallet, which is a different app than the nostr client.
As LN SDKs get better, it will be easier to integrate a LN wallet in a nostr app, which will solve this problem at its root.
We’re just starting after all
