the not picking the 1st year, is to do with letting the plant settle in and establish itself , and to let all the energy and nutrients that are in the leaves and stalks , return to the root system and build up strength for following years. You should do the same with comfrey .
Plants allowed to settle this way, will be stronger and last longer on the whole .
'They' are usually people that have grown it for many years , and know by experience what is the best way to treat the plant .
I have done it both ways , left it without picking any till it was really going well, 2-3 years , and , picked it first year . In the main ...'They' were right .
cheers
Russ