The first assertion author makes is that attacker is resource limited which is a pointless assertion for stack sponsored entities unless cost of attack is astronomical.
The second point author makes is that US elections are highly decentralize for attacks such as hacking electronic systems, making effective attacks very expensive. Again this is completely wrong assumption for two reasons:
(1) attacker doesn't need to hack all of US precincts. Thanks to thin win margins in elections, you just need to flip only few precincts that have been predicted as "middle". Typically these are just 1-5% of total so cost of attack is actually very low and could be targeted with precision for a desired result.
(2) There are only few brands and systems of voting machines and other electronic systems are actually in use. Again, you don't have to hack all of them. Just hack one or two and they might be good enough to slip 1-5% margin you desire. The connection to Internet is irrelevant. There are well known methods to hide malware in firmware drivers that could have been planted by a state sponsored hacker who obtained job at one of these companies making these machines. These kind of malware would be impossible to detect without very extensive reverse engineering. Think about this for a minute: For a cost of less than a million dollar (i.e. compensation for single hacker) you can flip election of entire country. It makes a huge economic sense to any adversarial government.
Overall, hacking political system has shown to have ROI of 10,000% or more and it should make a perfect economic sense to businesses and external governments. You can literally donate just $100K to political candidate and he would pass a law for you that makes your company 10s of millions for many many years. It never ends to surprise me that political donations by commercial entities and ultra-rich is not considered a crime in US. In most other countries they are known as "bribe". Even better, just flipping few of the precincts you can flip entire election in to your favor and you can do it all even legally by precision targeted social media. It's just amazing power to behold on one of the largest and most powerful country.