No they really didn't learn that because there was no unflavoured control.
They used one with nicotine and no flavour, one with favour and no nicotine and one with both nicotine and flavour. That's two with flavours, one without.
'Electronic-cigarettes (e-cigs) represent a significant and increasing proportion of tobacco product consumption' - eliquid contains no tobacco, they are not a tobacco product any more than NRT is. Nicotine is a tobacco product. It is extracted from plants in the tobacco family.
e-cig aerosols compared to conventional cigarette smoke - this study does not compare the two. - they are referring to a previous study. Perfectly acceptable science.
Conventional cigarette smoke has been shown to cause deleterious effects on oral and periodontal health [10]. However, the role of e-cig vaping and its association with carbonyl stress, inflammation, and DNA damage-triggered senescence on oral/periodontal epithelium remains unknown. - what thought process went into even comparing the two when the only thing they have in common really is nicotine content? - They are saying smoking causes this problem but there is no research yet as to whether e-cigs do. Again, a perfectly reasonable statement.
Just because the study is not constructed as you believe it should be, does not make it a poor study. They state what they are studying and report on it accurately. All the claims they are making in their paper are based on an accurate analysis of the data they have gathered.
My point is that vapers claim it is safe. Studies are suggesting that it is not safe. It is likely considerably safer than smoking. However, there is insufficient evidence to claim it is safe and until longitudinal studies are done, why should non-smokers be put at risk for the comfort and pleasure of vapers?
Oh, and I'm not a chemist. Never claimed to be one either.