How the Req derived at inverting input?
Step by step help:

You are calculating the effect of the input bias current IB. Where can that flow to and what voltage will that cause?


My understanding is that current will only able to flow through RF into the non-inverting input of the op-amp. since R1 will be at the same potential.

Have a look at my answer to OpAmp used as adder where I try to explain that the &quot;virtual ground&quot; will be not quite zero so current will, in fact, flow through R1. Now you've got two current paths. What is their combined resistance?
Respond in the comments and I'll update.