You're right in a strict sense. But in my experience such strictness is only useful in hard sciences and (maybe) legalese. There are exceedingly few things we can claim to apply everywhere, and even fewer we can "prove" to each other.
Give it a try if you don't believe me. Even categories we take for granted, like trees and fish, are not perfectly crisp, and "obvious" facts like "humans need a heart to live" have surprising exceptions.
> Pointing out that penguins can't fly doesn't make the case that birds can fly stronger in any way.
I disagree. It's such a common rule that there's a long Wikipedia page for the exceptions[1], and the first photo is of penguins, labelled "penguins are a well-known example of flightless birds.".
If I knew nothing else about the topic, I would take it as evidence that it's common for birds to fly, otherwise that fact would have been unremarkable. Not hard proof of a universal quantifier, but a useful rule nonetheless.