I'm glad you asked.
No, it is not safe. You might want to stop reading that source for information.
Do this instead:
Channel A on one side. Channel B on the other. Both grounded to bat neg. Use attenuators just in case of overvoltage.
Then use a math channel such as A minus B. High minus low side. This will provide you with the voltage drop across the injector and that is what you want to see. Deploy another channel for injector current.
Approach GDI in the same way.