I hauled fuel back in 2011 here and even delivered to the Hess, now Pilot in Carthage.
Running 15-20% ethanol will give you problems in a carb engine.
Do they secretly add it at the station? No. Does the driver when loading the tanker do it? No.
It's either blended from nozzles in the stream of sub octane gas going into the tanker at the loading rack or the E100 goes in first then the sub octane fuel goes in. It's all metered and the driver has no control of the formulation.
Before I hauled fuel locally the drivers for Mapco loaded E100 into their tankers manually then went to the main rack at Cumberland fuels in Nashville and got their sub. They had to calculate how much to put in there.
Last trip to Florida I stopped in Monteagle and Pilot didn't have E10. It's all E15. I really didn't need gas and didn't want to pay their high price for 90 octane non ethanol.