If the researchers had compared the groups only to each other and not to themselves, what impact could this have had on the results?

Let’s look at the use of ANOVA in the following nursing research article:
Özden, D., & Görgülü, R. S. (2015). Effects of open and closed suction systems on the haemodynamic parameters in cardiac surgery patients. Nursing in Critical Care, 20(3), 118–125. doi:10.1111/nicc.12094