As the article notes, the narrow focus on how actions are labeled be misleading. Some presidents issue directives as executive orders that others would issue as memoranda, and still others may accomplish similar goals by simply directing a given agency head to pursue a given course of action. Thus, these sorts of quantitative analyses are of limited use in trying to measure the degree of executive action.
In terms of the specific claims made in the article, the claim that Obama is “on track” to use PMs and EOs combined more than any President since Truman ignores the rate at which such actions are taken. As the data compiled by USA Today show, Presidents John F. Kennedy and Jimmy Carter were far more active, quantitatively, on an annual basis. A fairer quantitative measure than the absolute number of actions would be the number of such actions taken per year or per term, though this still would not take into account the extent to which control of Congress might affect the extent to which the president seeks to act unilaterally.
If we want to understand the extent to which a president is relying upon unilateral executive authority to advance policy goals, there is no substitute for looking at the nature of the specific actions taken, the precedents for such action, and the extent to which the president is acting pursuant to a valid delegation of authority from Congress. Such analyses take more work, and necessarily involve a degree of judgment that is not easily reducible to quantitative statistics, but it’s still the only real way to measure executive overreach.