Question about sequence of tenses

Latin Learning Latin
1

Eo opere perfecto, praesidia disponit, castella munit, quo facilius, si se invito transire conarentur, prohibere posset.

Imperfect subjunctive verbs are used with present indicative main verbs. Is this sentence an exception to the usual sequence of tenses rule?

2

The historical present may take a subordinate in the imperfect subjunctive. See ยง 485.e of Allen and Greenough / Latin Grammar.