Welcome to the Apple Support Communities.
I actually wrote that User Tip! I am glad that you found it helpful.
For an external SSD to function as a bootable device, the SSD and enclosure itself must be bootable. Some are not. The G-Technology G-DRIVE SSD series is.
As far as other SSD options are concerned, options are limited for a pre-USB-C iMac, such as the one you have. These models of iMac only accept USB-A. Some USB-C external SSDs, such as the G-Technology G-DRIVE SSD, include a USB-C to USB-A adapter cable, but recently, there has been a trend to remove this older-style cable. This includes LaCie SSDs, some SanDisk SSDs, Crucial external SSDs, etc. And while you have this connection limitation, you still must look for an external SSD that functions properly for a bootable device.
So, you need a bootable external SSD, with a USB-A cable included, and something that is high-quality and very reliable. This is why I like the G-Technology G-DRIVE SSD series. The only con I see with it are the included cables are shorter.
I hope this helps!
Cheers,
Jack