It does start at 2-3K, but it's very shallow. It's only down <0.5dB at 20K. At 10K, it's down less than 0.1dB. You must have better ears than me to hear it.The "treble cap" is indeed audible. The LPF in your graph already starts between 2-3k.
This cap works well with AC701 since it has a very detailed and excited top end. Other tubes can sound very dull and dead with it.