The correct interpretation is that the course is measured following the shortest possible route (SPR) within 30 cm of cones, curbs, barricades etc. It almost matches the way tracks are measured except the exact measured line on a track is 30 cm from the curb (if the track is made to have one). Theoretically a runner could run within that 30 cm on a course and thereby shaving a little off the measured distance just like a 200 m runner could do by running next to the lane line (fyi- the measured line on a track constructed not to have a curb or those in lanes 2+ is 20 cm from the outer lane line of the lane to the left). For a road event, this is often impractical considering that the edges of the road near curbs are often not in the best condition. But a better example might be race walkers who hug the turns. In that case just like a road running event, the course is measured 30cm from the cones. Some measurers would measure turns virtually rubbing against the turns making it almost impossible to run less then the distance. BUT, there are measurers out there that may not always measured the absolute tangent. This is the critical aspect of course measuring - measuring the SPR.
In regards to the 20 meters - That's the short course prevention factor that is added to the half marathon distance (should be 21 m). All courses have a Short Course Prevention Factor (SCPF) of 0.1% added to ensure that if a record is set, the course is found to be at least the distance upon a verification re-measurement. As previously noted, 5 m in a 5k, 10 m in 10k and for the half marathon (21097.5 m) it comes out to 21 m.- I was rounding.