Yeah "serious cyclist" feels so impossible to define that it's meaningless. If I ride to work every day but go to a shop for maintenance instead of buying my own tools am I not serious?
We've seriously shifted the goalposts from "serious cyclist."
Folks who are serious about any hobby or activity are also often relentlessly practical. I used to ride 100km from the city I lived in, out to my brothers farm. It was not a particularly long or challenging ride, and it had cell service most of the way, even 20 years ago. I always did a gear check, and carried spare tires, but in 4 years of doing that 20-30 times per year, I replaced tires, but never a chain on that ride. I also never carried the tools to do that work because it fell into the category of unnecessary gear. If there was a problem, I was only about a 30 minute wait for help from either side if I needed to be picked up.
I also used to do a fair bit of trail riding, and for a much longer period of time, and for significantly technical trails. Again, a gear check ahead of time, and proper maintenance meant I took tubes, but not the chain repair tools (and one time I regretted not taking 4 tubes, because it was a long and hot 14km hike back carrying my bike on a July afternoon).
I am not going to gatekeep your ability to gatekeep, but you should at least understand that your perspective on what constitutes a "serious" cyclist is completely subjective and not rooted in anything meaningful.
There’s a tendency to ascribe value to words like “serious” but it’s a temptation, nothing more. Who cares that some bozo thinks “serious” drivers drive manual transmission cars, or ride bikes in a particular way.
The worst mechanical I ever had was a rear derailleur which deformed under load and pulled into wheel. I have never heard of anyone carrying a complete spare derailleur unless on a round the world trip.