Is a postdoc-interview presentation a norm?
At least in my field and country, giving a talk about your previous work is pretty common for the hiring process of any academic position. But that does not mean you should prepare a talk.
Nobody can seriously expect you to know this and automatically bring a talk to a job interview just because it’s the unwritten norm (and you probably do not want to work with the people who do).
Moreover, it is impossible for you to deduce the desired length, audience, scope, and other aspects of the talk.

Instead, you have to estimate how likely it is that this aspect was just forgotten in the invitation (e.g., going by the other details it contains).
If you think that there is any non-negligible chance that this is the case, just ask them.