Let's look at the Full Statement where that 5% Statistic was mentioned.
"E-cigarettes and long-term harm - the possibility of some harm from long-term e-cigarette use cannot be dismissed due to inhalation of the ingredients other than nicotine, but is likely to be very small, and substantially smaller than that arising from tobacco smoking.
With appropriate product standards to minimise exposure to the other ingredients, it should be possible to reduce risks of physical health still further. Although it is not possible to estimate the long-term health risks associated with e-cigarettes precisely, the available data suggest that they are unlikely to exceed 5% of those associated with smoked tobacco products, and may well be substantially lower than this figure."
Promote e-cigarettes widely as substitute for smoking says new RCP report
I don't think the RCP is giving Carte Blanche for any e-Liquid Ingredient to be Vaped under their "unlikely" / "may well be" naked 5% Statistic.
If a person wants to use e-Liquids that contain High Level of Diacetyl and or Acetyl Propionyl, that's cool. Just like if a Person wants to Vape at High Wire Temperature such as above 450F, that's cool also. It's all a personal choice.
All I'm say'n is I think this "not possible to estimate", "available data suggest", "unlikely to exceed" 5% RCP statistic can give some Vapers a False Sense of security that anything they inhale is Hunky Dory.
And I Don't believe that is what the RCP was trying to state.