This is incorrect information, you're interpreting the frame data wrong here. To determine the gaps in between the strings you take the cancel advantage on block and subtract it from the execution frames. With the case of the 21 string here is the data about the cancel advantage on block:
2,1 - +18
2,1,2 - +28
Here is the data on execution frames:
2,1 - 18
2,1,2 - 24
You can't poke in between the first part of the 21 string (so there is no gap in between the first 2 and 1), however, in the case of 212, the 1 and the second 2 has a 4 frame gap allowing you to use parries, xrays, moves with armor and invincibility (like Cage's Ex-np, Raiden's teleport) to get through the gap.
21 pressure isn't entirely that bad, but the major flaws in it is you can poke SZ before he does the 21 string and sometimes the 2 whiffs on low hitbox characters. Another thing is if you do 21~D3, you can potentially be crossed over and be blown up.
Best thing is either end the string completely into clone or do 21 and block low against a poke attempt, then counterpoke back. The 3rd option is to do 21 and then walk back in an attempt to whiff punish, but this requires conditioning on your behalf.