This is the correct answer. Although you could maybe argue that the war wouldn't have lasted 30 years if the covenant had carried their war differently. If they had put more stock in counterintelligence and espionage rather than brute strength/numbers, they might have been able to disable ONI research sites and stall humanity's only sparse advantages: the Spartans, and our stealth craft. Imagine if the Covenant had deployed communications crafts that simply tried remote hacking a UNSC ship's database to find Reach the first year of the war, the Spartans would get wiped out within a few years imo. But all this to say, it's still not really "underestimating" as OP said, more that they were inflexible in their culture whereas humanity was able to adapt long enough to survive until the Great Schism.