It's funny you caught that. I was playing around with ideas and I couldn't decide if I wanted to go with an existing event type or create a new application event type. The reaction event was the simplest event type I could think of at the time.
Discussion
In my mind Apply for a bounty was something much more significant than a 'like'. And I wonder if you would eventually introduce some attributes of an 'Apply' - like estimated timeline etc, then it should definitely be a separate kind.
I have some updated thoughts here: https://github.com/nostr-protocol/nips/pull/865 and I would appreciated any other feedback.