Avoid abbreviations such as “i.e.” and “e.g.” (Many people don’t know what they mean.)
The problem with this statement is that it is neither quantified nor backed up with any research. How many is 'many'? Is it 70% of people? 20% of people? 1% of people? The word many just indicates a guess by the person who wrote the article.
Maybe the writer has met one or two people that didn't understand and has generalised to the whole population based purely on his/her own experience.
The truth of the matter is that there are many people who do not know what 'abbreviation' means, that don't know what a document is or don't know what a sentence is.
You have to draw your own arbitrary line according to how educated you think your readers are. I am sure that anyone answering here understands the abbreviations. So does anyone who has a reasonably good education.
On the other hand, it doesn't take a lot of effort to write things in full. Unless of course you repeatedly have to use the expressions in the same document.