It really isn't.
The definition of engineer is "a person who designs, builds, or maintains engines, machines, or public works."
We're building software engines, machines, and public works. Just because we're applying it for the software incarnation of the concept doesn't break the definition.
This is getting silly.
How isn't that like claiming you're a chef because you have a catering company where you serve microwaved foods, and claim it's silly to try to distinguish degrees of cooking skills.
By the definition you gave, no one can be a chemical, industrial, or electronic engineer. Silly!
In doing so, we see what lessons there are to take from each other, and gain a better understanding of both fields. For example, the author cites that version control is an innovation that traditional engineering could hugely benefit from.