Flashtool Not A Sin File -

The confusion likely arises from workflow. When a user downloads official Sony firmware (an FTF bundle), they are actually downloading a collection of SIN files, compressed together. Flashtool then processes those SIN files: it decompresses them, verifies their integrity, and extracts the raw binary images to be flashed to the correct partitions. Flashtool may even create an FTF archive from SIN files. Thus, a user might see SIN files listed inside Flashtool's GUI or file directory and erroneously conclude that Flashtool is a SIN file, rather than recognizing that Flashtool is the interpreter reading them.

No. GSIs are provided as .img or .xz files. Flashtool expects SIN. Use Fastboot or TWRP to flash GSIs. flashtool not a sin file

If you have a custom kernel or recovery as an .img file and you must use Flashtool to flash it, you need to convert it to SIN format. This is rarely needed (since you can use Fastboot instead), but possible. The confusion likely arises from workflow