Looks like a 12v addressable LED IC with a redundant/backup data feed, and a standard RGB 5050 LED. Probably the biggest challenge will be matching the 5050 LED, as different manufacturers will use different ones. To be honest, your most practical approach will just be to buy some from a few places online and see how it goes - then when you find the exact one you want, stick with it.
Well that post on the Aussie Christmas Lights forum is from 2018, so they aren’t that new. I’d say there’s a good chance that’s them. Maybe try and see if there are any markings on the black chips - you might be able to read a part number off them.
Unfortunately, Core doesn’t stock any WS2818 strips. I couldn’t find a comprehensive spec on the protocol either but I imagine it close if not the same to the WS2812’s which would make the main issue the supply voltage (From what I found Bin and Bout are backup data lines).
I don’t have experience in sourcing WS2818 strips so won’t be able to vouch for any particular suppliers.