Which person?

I read Michele Berdy's translation column in the Moscow Times every week. Generally she's very engaging and knows what she's talking about. But occasionally, she slips. For example, last Friday's column, on qualifying your statements:

For example, if you are discussing something theoretical, you can use the third person plural forms of допускать (to admit, allow, concede) or предположить (to suggest). Предположим, у нас все стали идеальными чиновниками (Let's say that all our bureaucrats become ideal civil servants). Допустим, перестали воровать (Let's hypothesize that they stop stealing).

You certainly can use this form. But it's not the third person plural.

It's the first person plural, in a form sometimes called the 'first person imperative' and sometimes the 'hortative' and almost always translated by "Let's V".