I enjoyed the article, but I worked for a startup in 1999 that was funded by a technology accelerator. So it would appear Ycombinator wasn't the first, unless this is meant in some more specific way.
I've known of many others too. I'm sure Red Hot wasn't the first.
The pre-YC incubators operated on a completely different model. They typically had shared services, centralized control, and were rarely filled with entrepreneurs doing their own thing.
"Accelerator" was a term that YC used to distinguish themselves from that model, and whether or not they were the first to use the term what they did was truly different.
Just an example: CMGI, Inc. was calling themselves an Internet Incubator around 1995.
Probably better to make a claim based on success.