Why did my vet give Convenia to my cat for non-skin infection?

While it is true that Convenia is labeled for the use of skin infections, it is also used off label for other diseases in pets, such as upper respiratory infections, UTIs, or bacterial infections. I would caution about what you read online about it, as many of things you read may be sensationalized, exaggerated, or simply untrue. Drug use in animals is closely regulated, and if there was a concern with a particular drug killing animals, then it would not be used. Any drug has it's side effect, and any drug can cause an allergic reaction in an animal potentially leading to death, but it's a good drug to use when you want an animal to have antibiotics in their system for up to 14 days without having to worry about getting an animal to ingest pills. If you do have concerns as to why your vet chose to use this drug on Frida Katio, I suggest you speak to them, and they can explain why they made this choice. Unfortunately, without seeing her in person, I can't say if this was a good choice or not, so speaking with your vet is the best option for you.