It was me who said that, and I'll say it again.
You absolutely can be as forthright and rude and blunt as you like with other people's children.
How else are they to learn?
Are you just going to fume impotently and expect them to understand what you want from them, behaviour-wise? They are not mind-readers. They often need direct instruction. In the case of two year olds, the more direct the better. With two year olds, you teach them how to treat you or your baby or your possessions or depending on personality and how much work their parents have put in, they will do things that you are going to fret about - break things, dirty things, spread germs, hurt a baby unwittingly, etc. If you don't want these things happening, then speak up, and don't hint or be indirect. You are not going to hurt their feelings by being blunt or fortright or saying something directly like a firm 'No, no, no - you can't touch', that might come across as rude to an adult.
It's not nasty to tell a two year old exactly where she is going wrong and exactly what behaviour you expect from her. If her mother hasn't bothered, and your baby stands to pick up a cold or gastro enteritis or whatever else the two year old may have lurking on her fingers, them rude, forthright and blunt are exactly what is required.
I don't understand how you think that is nasty. I haven't either stated or implied that two year olds are horrible in any way. I have implied that two year olds are in need of direct instruction, and I am right. They do not come equipped with innate knowledge of how to treat a baby or a baby's toys, or how colds are spread.
The handwringing over how polite you have to be to a two year old is teddibly British, and incredibly silly.
There is no point letting a two year old do what comes naturally because you are too timid to intervene, and then getting so furious about the predictable result of letting her have free rein that you feel like shouting at her.