Yes there are alot of things on the internet back then. When it was an closed system of trust.
But like any system that opens up new people are more then willing to take advantage of trust. Ergo we are in the zero trust era where everyone needs to prove. They are not a malicious actor before they will be listened to.
It's apparent it sees the SD card, but it's not seeing the MBR on it. I doubt the DOS version is the issue, because it's not event attempting to boot.
Unfortunately there's no guarantee your 360kb floppy drives are in working order if you don't know already.
I usually recommend a flash floppy but for an machine like this you're really getting into the weeds with it not being bootable.
Try an different SD card, they can be weird, smaller is better.
And try writing a different dos bootable image to it.
I always drive in Maximum Over-Reich to conserve gas