This was a linkpost, I didn't write that paper.

I'm not sure how you're defining nihilism there?

The term date from at least 2009.

MacAskill:

Up until recently, there was no name for the cluster of views that involved concern about ensuring the long-run future goes as well as possible. The most common language to refer to this cluster of views was just to say something like ‘people interested in x-risk reduction’. There are a few reasons why this terminology isn’t ideal [...]

For these reasons, and with Toby Ord’s in-progress book on existential risk providing urgency, Toby and Joe Carlsmith started leading discussions about whether there were better terms to use. In October 2017, I proposed the term ‘longtermism’, with the following definition:

Yes. One of the Four Focus Areas of Effective Altruism (2013) was "The Long-Term Future" and "Far future-focused EAs" are on the map of Bay Area memespace (2013). This social and ideological cluster has existed long before this exact name was coined to refer to it.