I don't get it

Written offer under 3b) has the problem Matt Domsch already cited which is a 3 year time
period for which you have to maintain the equivalent source packages. When exactly the clock
starts ticking for the 3 year time period is fuzzy and is a considerable risk if adversely
interpreted. 

Fedora Project having to retain all the binary and source packages for every update it has
every released (since different spins have different package sets and package versions) which
is a pretty huge amount of storage space considering the release and update cycle. A
comprehensive solution is being worked out on that problem as indicated in the announcement. 

https://fedorahosted.org/correspondingsource/