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.
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.
CMGI and the other “incubators” of that era were nothing like YC. The label “accelerator” was applied to YC years after it was started because of the need for a generic term to describe YC and all of the clones. Whether or not that label was previously used for some other kind of business is irrelevant.
Makes sense. I suspected that might be the case, which is why I mentioned the possibility of something more specific intended. (In case other readers are struck by the same thought, it might be worth a footnote).