Current noise as spectral density from given RMS noise
What current noise will you get from a 10Meg Ohm resistor in 1MHz bandwidth?

1KOhm is 4 nanoVolts/rtHz

100Kohm is 40 nanoVolts/rtHz

10Meg Ohm is 400 nanoVolts/rthz

Current noise density is 400nanoVolts/10MegOhm = 400 e-9/1e+7 = 400 e-16
or 40 e-15 or 40 femtoAmps/rtHz.

Scale up by sqrt(1 MHz), and find 40fA * 1000 = 40 picoAmps.

But............this is the current noise for a single resistor.
That Keithley has numerous circuits involved in implementing a programmable current source.