Why can the offset voltage of the comparator let the transition time be longer or shorter?
If I added a small offset voltage in series with one of the comparator inputs like this: -



Wouldn't you now expect the comparator to switch and select CV mode when \$V_{FB}\$ is greater than \$V_{REF}\$ + 5 mV?

The 5 mV is, in effect, an input offset voltage.

This would mean that the time taken in CC mode would be a little longer because the battery terminal voltage has to rise to a slightly higher voltage in order to make the comparator switch to CV mode.

The actual transistion time in switching between CC and CV mode is unaffected - it will be rapid (sub microseconds) - it's just that the point in time that the transition occurs will be a little later in the scenario above. If the 5 mV was reversed, the transistion would occur a little earlier in the charging cycle.