[nradov]

The best evidence we have indicates that most recovered patients will retain a significant level of immunity for at least several years.

https://www.jimmunol.org/content/early/2020/09/03/jimmunol.2...

As with any pandemic there will be a small minority of outliers who experience symptomatic reinfections.

[dhimes]

The paper you cited doesn't supply evidence- according to the abstract it's their best guess based on experience with other coronaviruses.

The evidence is actually scant but there's a decent discussion of it in the Guardian article I linked below and the paper linked from it.