When you're dealing with science, it's more strict than that. It's not a logical fallacy. It's a matter of engineering.

I think what he(?) was pointing out simply that the lack of design intention doesn't mean lack of functional capability. Certainly my fingers weren't designed to type, yet there it goes. The other side of such a fallacy would be akin to stating that because I can type with my fingers, they must have been "designed" (intelligently?) to type. So abstractly, I have to agree (since it was posed in an abstract variable replaceable way).

In this case, however, since the scientists know what they'd need to "prove" life, they can state that Curiosity doesn't have enough sensor capability to gather such evidence. Rather than stating that Curiosity was not designed to detect life, it would be more appropriate to state that Curiosity is incapable of providing rigorous proof of life.

Now, back to the evidence. Stating that what Curiosity has found is not evidence, may be taken at face value only if we narrow the definition of "whatever Curiosity has found" to the information capable of being gathered and returned, as opposed to the sources of the exhibits under observation (which may or may not have a biological origin, or may or may not be a precursor to biologic functions). A thin metaphysical/epistemological distinction to be sure, but one that is relevant because when we need to trust our senses, we need to know our sensor limits, and when we need to trust remote sensors, extra-terrestrial remoteness is as remote as it gets, and its hard limits cannot be easily overcome once deployed.

Typing was designed for your fingers though. Or some abstract idea of fingers anyway.

Curiosity is loaded with sensors, these can be used to draw conclusions they weren't designed for. Here's an admittedly absurd example: if Curiosity's cameras recorded a Martian puppy running around it, scientists would almost certainly consider that conclusive evidence for life.

It would be evidence but not conclusive evidence. In this case, we might think it was some sort of horrible prank played by a disgruntled scientist who hacked the instruments. But it might be enough for a second mission designed to conclusively verify the existence of life.

What if the Martian puppy started licking Curiosity and started to reprogram the cameras and antennas to communicate with us like an interplanetary telephone system?

That would be a pretty conclusive proof of live even though Curiosity wasn't designed to work in that way.

That's not intelligence, it's instinct!

The ability of a device to detect "life" depends on the assumptions about what types of life one expects to find and what level of certainty is required.

Curiosity could certainly detect certain kinds of life... but not the types of life astrobiologists are guessing might exist near the surface. Curiosity definitely can detect active chemical processes for which there would be few if any non-biological explanations... but after so many false positives in the past the science community has decided that alone would not be definitive proof.

Curiosity was not built to detect life, but it can. However scientific conservatism would likely mean any “yes” result would be interpreted as a ”maybe.”