No, I think it should be a symbol only.
Periodicity can be 1, 2 or 4 slots, so the duration of PDCCH can be 14, 28, 56 symbols.
And the 1-time duration can be 1, 2 or 4 symbols.
So with 4 slots monitoring = 56 symbols.
And duration with 4 symbols, the PDCCH search window will be 4 consecutive symbols and it will be represented with DCI bitmap.

Then what would be the reference point for UE to decide where to flush the symbols?
I was taking symbol.on which dci 2_1 as the reference point and 14 symbols backwards from it.
Excluding the uplink symbols.

It is in the same reference you shared.
Code Block Group Flush Indicator with boolean.
DCI Format 2_1 is used to signal up to 9 sets of pre-emption indications.
The single set uses 14 bits to represent 14 pre-emption indications, i.e. DCI Format 2_1 has a maximum payload size of9 x 14 = 126 bits.
Why are you referring UPlink symbols? Any reason?