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 tense.
servi fugiverunt ne poenam darent.
dux militibus imperavit ut urbem aggredirentur.
puer timebat ut puella maneret.
In all of these cases the secondary clause is not a real event, but rather an intention, or a worry, or a purpose, or some such thing. With result clauses the result clause is something that actually happened, will happen, is happening etc. The subjunctive mood is simply a result of the construction, but implies no unreality. In fear clauses, indirect command, purpose clauses etc the secondary clause is in a sense unreal, a possibility, and hence the tense is not as important as in a result clause. The tense follows a more general rule - the sequence of tenses.
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