> Experimental group of mice received subcutaneous injections of the LD50 dose of (−)nicotine hydrogen tartrate of 3 mg/kg/day, 5 days per week for 24 months, and control group received the vehicle phosphate-buffered saline.

> ...

> The purpose of this pilot study was to evaluate the pathobiologic effects of long-term, high dose nicotine treatment of A/J mice susceptible to tobacco product-induced lung carcinogenesis.

These were mice genetically engineered for increased cancer susceptibility and also given insane dosages. This study is designed to answer whether nicotine is not carcinogenic. That is, if the result was negative then it would suggest any future study would be wasted effort. A positive result doesn't say anything about the safety of nicotine, per se; just that there remains the possibility it may be carcinogenic. That's very different as many absolutely benign substances could also produce this result.