I believe they are correct. While there are smaller naturally occurring mutations, there have been no major structural changes that would mean a reinfection that I’m aware of. Now, while it’s uncertain how long the immunity lasts, the body generally does do a pretty good job of viral memory.
Yup, if true, this is the best news yet. This also means odds a vaccine will work becomes a lot better. Probably doesn't have a huge impact between now and a vaccine but it does mean this wont be a yearly recurring virus like some have feared.