Yes, that is a very good explanation. Result clauses do not
follow the so-called sequence of tenses as other similar clauses
do. This is because result clauses are felt to make the most
sense when using the most applicable tense. Think of it this way
using your examples: 'So numerous were the enemy that they
easily took the city. Tot erant hostes ut facile urbem caperent.
Tot erant hostes us facile urbem ceperint.' The result clause is
a clause that describes a separate, independent and real event
from that of the main clause. IT is a related event, a resultant
event, obviously, but a separate and event nonetheless. The
imperfect tense in this case does not give a sense of the
finality needed here. It would mean something like 'So numerous
were the enemy that they were easily taking the city.' The
incomplete sense of the imperfect anticipates some sort of
interruption. Compare this with a purpose clause, indirect
command or a fear clause, both of which demand the imperfect
subjunctive following a historical t