I think the aircraft being familiar makes it worse: if you're used to going through a certain motion to do a thing, it may be one of things your brain can do without really thinking about it much, which is where the danger comes in.
I've engaged my wipers when meaning to shift gears before, in my truck which has a steering column shifter. After driving the truck for years. I have ADHD and I very often let my brain go on autopilot for things I do every day, and sometimes it just does the wrong thing. It doesn't matter how complicated or "intentional" the task has to be: my brain will memorize it to the point that it can execute it on its own without me consciously thinking about it.
I think it's totally plausible it was a muscle memory thing, if the at-fault pilot's brain works anything like mine.
(Side note: I actually took some flying lessons, including going through all of ground school, and realized that my brain is just not cut out for flying. I am the type of person to "cowboy" things if I feel like they're not worth doing, and flying is an activity where the tiniest missed checklist item can result in death, so I realized I have a statistically high likelihood of crashing due to some boneheaded mistake, and stopped taking lessons.)