It is normal for sims to gain skills in gardening when they do gardening jobs like harvesting. The description of this scenario doesn't say they can't gain skills just that it is harder.
The last time I did this my Sim had a huge garden and did not obtain any skills. My current Sim is also now level 3 handiness. It just wasn’t what I expected. I did another one where you are supposed to raise a child from infant to teenager and the scenario ended while the kid was still a child.
Thank you for your suggestion but I have never played with mods or cc.
