Do SSDs actually lose data when off?

Yes. And more dense SSD lose data faster. It's just physics. Charge from isolated gates of flash memory leak due to non-ideal isolation and tunnel effects, so, earlier or later all charges will disappear. And smaller structures on crystal makes things worse. To be shure data will be OK you need to connect SSDs to power every week for few hours, SSD will run charge maintaining process.

read that was a misinterpretation and that only becomes true if an SSD is at end of life and already dying.

It is about powered SSDs. Leak currents become larger with usage, and at some moment even powered SSD charge restoring process will not be able to mitigate that leaks. But even completely new SSD will lose data without power in relatively short time, from few years for old, low capacity SSDs made with process with high nm and months for new, current SSDs made with modern few nm process.

Same with USB sticks. Some old USB sticks may be could withstand many years, but I will not bet that new ones will last a year on a shelf.

In any case flash based storage is vulnerable to data loss and should not be used for critical data. HDDs, tapes and one-time recordable CDs/DVDs (not rewriteable ones) orders of magnitude more reliable for that task.

Do SSDs actually lose data when off?

Yes. And more dense SSD lose data faster. It's just physics. Charge from isolates gates of flash memory leak due to non-ideal isolation and tunnel effects, so, earlier or later all charges will disappear. And smaller structures on crystal makes things worse. To be shure data will be OK you need to connect SSDs to power every week for few hours, SSD will run charge maintaining process.

read that was a misinterpretation and that only becomes true if an SSD is at end of life and already dying.

It is about powered SSDs. Leak currents become larger with usage, and at some moment even powered SSD charge restoring process will not be able to mitigate that leaks. But even completely new SSD will lose data without power in relatively short time, from few years for old, low capacity SSDs made with process with high nm and months for new, current SSDs made with modern few nm process.

Same with USB sticks. Some old USB sticks may be could withstand many years, but I will not bet that new ones will last a year on a shelf.

In any case flash based storage is vulnerable to data loss and should not be used for critical data. HDDs, tapes and one-time recordable CDs/DVDs (not rewriteable ones) orders of magnitude more reliable for that task.