Merkel and other world leaders were likely talking about herd immunity (they may have walked that back now) but we don't know if the immunity from the infection is long-lasting or not. For instance, it is said seasonal coronavirus immunity lasts less than a year so they're not particularly immunogenic.
Furthermore, the virus has several mutations and we don't know if exposure to one mutation is sufficient to produce immunity against another. Given that's it's not particularly immunogenic it seems it won't.
When people are infected with OC43 and HKU1—two other coronaviruses that regularly circulate among humans and cause common colds—they stay immune for less than a year. By contrast, immunity against the first SARS virus (from 2003) holds for much longer. No one knows whether SARS-CoV-2 will hew to either of these extremes, and according to one recent study, its behavior could mean anything from annual outbreaks to a decades-long quiet spell.