This was brought up recently here and while I can see how disappointing that may be there are a few things I wanted to bring up.
1. No, this doesn't mean Artemis is an illusion. In fact Artemis should be the least affected by this reveal. Yes she is an Amazon but she's part of the Bana Mighdall. This hasn't been delved into in RHATO but previous canon had this group of Amazons separated from Diana's Amazons centuries ago.
The Bana Mighdall chose to worship the Egyptian Gods instead and they actually reproduced which the women of Paradise Island couldn't due to their immortality. Artemis herself was one of the new generations of Amazons. But given what Lobdell said that seems to have changed as she's immortal and older than she looks. Regardless this wouldn't affect her.
2. I think this would affect Donna Troy a lot more since she was "made" by the Amazons. But given the fact the Titans know her it seems unlikely that she's fake. I haven't been following so I don't know if this is all being tied in together.
3. This is all an illusion to DIANA and whoever she brought with her to the "island." Steve Trevor is the only one she's been shown to bring that would know the difference. This might be the case for other exiled Amazons like Ex from Demon Knights but that hasn't been dealt with yet.
4. The thing that bothers me the most about this reveal is that it means Diana was fooled for years despite being the representation of truth. That really feels off to me.