just for interest: what chip do you have on your i2c backpack?
It is a PIC18F13K22. Should definitely handle 100kHz I2C.
That's the part that is no fair. The data sheet that you posted lists the max I2C speed at 400kHz and no minimum. I have used chips that told you that 50kHz was the max I2C speed (even 10kHz during initialization), but telling you it's good to go at 400kHz and then...... no fair.
BTW: I wonder why the scanner found it - does that one use a variety of I2C speeds?