Thats really curious about alcohol in msr bottles, they are aluminum, so that surprises me. I will look into it though.
and yes, there are real dangers using soft drink bottles or anything 'drink" looking, I was the only one using it, but still was concerned and did mark it like crazy.
the reason that the MSR fuel bottle isn’t rated for alcohol is because isopropyl alcohol dissolves the o-ring seal on the lid which will cause a leak. Also I’m not sure what plastic is used for the MSR lid, but most plastics either resist petroleum based fuels or alcohol, but rarely both. Since the MSR bottle was designed for kerosene, white gas, diesel fuel, and gasoline, I’m pretty sure the cap’s plastic is not able to withstand the properties of alcohol.
When Ethanol was starting to show up in the gas stations, there were lots of problems with fuel line deterioration and carburetor problems due to the alcohol drying out the rubber. The fix was to switch to Nitrophyl plastic floats, and all the rubber was replaced with Nitrile Rubber.