more than likely this warning is because typical extension cords are 14/2 with a ground. They can heat up quickly under a load like a high amp humidifier would draw.
Dehumidifier says not to use extension cord
DoItYourself.com
DIY Home Improvement Information | DoItYourself.com
Aug 14, 2017 — Cords can be dangerous for many reasons, which is why manufacturers don't want people to use them. However, they are fine for temporary use.
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Then there are extension cords that can handle the amps.
How many amps can 12 2 extension cord handle?
12-gauge: This gauge is suitable for heavy-duty use, such as powering large power tools and appliances. It can handle up to 20 amps of current and is suitable for cords up to 150 feet in length. May 2, 2023