The article misses a major point here which explains much (though not all) of the trend: household sizes have decreased dramatically in the last 50 years, and the number of single-person households has increased dramatically. Single-person households can generally be expected to have at least one bathroom per person, and small households will generally have higher ratios than large households.
In 1960 single-person households were about 13% of all households; they have more than doubled, to about 28% in 2019 [1]. In 1960 the average household size was about 3.3; in 2019 it was 2.5 [2].
[1] https://www.census.gov/content/dam/Census/library/visualizat...
[2] https://www.census.gov/content/dam/Census/library/visualizat...