The thing about having birds x 60 years or so is that today's modern chemicals weren't around in the past.
Yes, any plug in, room spray or even carpet freshener is dangerous to birds. Even many scented candles. Self cleaning ovens are killers too and toaster ovens.
These birds were too young to succumb to dietary issues unless they ingested a toxin.
Here's the thing about today's bird seed diet: Many makers spray 'nutrients' onto the seeds so they can claim on the package "nutritionally complete" or whatever. Lie, lie, lie.
Seeds are like feeding a toddler nothing but ice cream and french fries while keeping them in their bedroom for life.
Initially it all looks good, the kid is growing and seeming to thrive, right up until the diabetes, heart attack, kidney and liver failure, tumors, you name it.
Birds are the same. Most on all seed diets have a shorter lifespan and nearly all will succumb to liver disease (Hepatic Lipidosis)
Next, few people know that grit and/or sand is absolutely unnecessary for hookbilled birds (like budgies) - and cause impactions and malabsorption resulting in malnutrition.
Finally, yes, the carpet cleaning could also be behind it. Chemicals used in today's cleaning products are different from years ago and I always insist that people move the birds to a very remote location in the house where the door can be shut and preferably a window open, even just a little.
After the carpet cleaning, open windows and wait 48 hours before bringing birds back in the room.
Or just avoid cleaning with commercial products and stick to formulas with natural ingredients like baking soda, borax, vinegar that you make yourself.
Please do not feel guilty. You didn't do anything on purpose and how on earth should you know what you don't know?
I'm about to post a diet hand out sheet to start out right with your next birds, so again, hang in there for me ok ?