There could be DC leaking from the iPod.
DC could upset whatever you are going into.
20K will bleed-off some DC and the audio loading is insignificant.
The 470r resistors should match much better than 10% for 1dB unbalance. 5% is wise. 2% is now cheap enough. 1% seems overkill to me. I'd probably grab five of ANYthing 220r to 1K, use the meter to find two within 3% of each other.
There is NO need for the 20K to be 1%. Anything 10K to 100K is fine. Looks like this author has a drawerful of 1% parts.
99% of the time, you could omit the 20K. Old sound guys learn to defend against the 1% bad-days, especially if it only costs $0.12.