Kea vs ISC DHCP timers
ISC DHCP uses 3 values (max, min and default) values for lease-time (valid-lifetime) in Kea). These 3 values are in the Kea code (aka the triplet class) but are not reflected in config. (note I don't say a solution is better but they are different). As the valid-lifetime is a mandatory config parameter this means Kea is rigid (same comment).
For other timers ISC DHCP has preferred-lifetime but derives t1 and t2 (aka renew and rebind timers) from the valid one using standard formula. ISC DHCP follows more the client query, i.e., it uses configured values including computed values only as default and bounds.
This is not a call to change something: it is just a summary for documentation and some infos in the case a customer requests more flexibility.