Although some sugars are worse than others (e.g,. hard candy which sticks to your teeth), just about any carbohydrate you might eat will contribute to plaque (including fruit, vegetables and even whole grains) and indirectly to increased susceptibility to dental caries (how hard your dental enamel is and how acidic your diet is are other contributing factors)...
You can vilify "refined" sugar and HFCS industries all you want, but equivalent sweetness of organic molasses or maple syrup are probably worse when it comes to plaque contribution to dental caries...
FWIW, the existing types of studies that target limiting sugar are not correlation studies, basically they are ecological survey studies on population statistics by estimating their sugar intake vs prevalence of cavities at certain points of time. The author was suggesting specific studies that try to address correlation between certain foods and cavities were being suppressed by the sugar industry.
But back in that time frame of the 60's and 70's people were looking for a vaccine for plaque so interest in such studies may have simply dissipated w/o needing a big conspiracy. This is probably due into a large part of seminal studies on dental caries (in the late part of the 19th century by Miller) that established the link between enamel decay and acids produced by plaque bacteria fed by potato starches (not sugar because it wasn't a wide part of the diet) and later studies in the 1940/50's (by Gustafsson) seemed to indicate the frequency of use of sugars (rather than the quantity of sugars consumed). By the 1960's/70's there were already studies (like Duggal) that implicated snacks like cakes and biscuits (that combined glutinous starch with sugar) which were consumed at higher frequency (more than 3 times per day or basically outside of mealtime) had serious "cariogenic" potential.
Unsurprisingly, this research wasn't suppressed, but basically ignored by the government panels. Sadly, back in the '70's, it was much easier to pay off people to simply ignore research, than to actually suppress the funding to start research (not knowing the outcome).
On the other hand, some of the unrepeatable history of dental research sponsored by the sugar industry, such as the one in Vipeholm, Sweden already yielded habits like lördagsgodis. I'm sure the sugar-industrial complex in the US would love for something like that to pop up in the US (and not just restricted to Halloween)...