"Assuming" means an assumption is being made. And an assumption is a proposition that is taken for granted, as if it were known to be true. Don't demean it (blah, blah, blah, etc.)

Gee, thanks sensei.

So I'm not allowed to ask where they came up with the number 1000? If I ask such a question, I am somehow 'demeaning a classic tool in science'? I should just take it on faith that its a well founded assumption because you directed me to a Wikipedia page that has nothing to do with the original article?

Get over yourself.

Assuming the average communicating civilization has a lifetime of 1,000 years

From what orifice was this number pulled?

They have a sample size of 1 civilization with an unknown end-date of communication capability. Where does 1000 years come from?