Thanks.
I did notice the typo but figured you would realize quickly.
You have it right. the ! means not, opposite to what the function states, at least in a well named function like these where it reads like a phrase “if is null” but we change it to “if is not null” by adding ! before is_null(). You understand it fully from what you wrote.
Also…
What I done was mentioned a couple of different approaches. One is to put the result from get_the_content() into variable. This uses memory and should really only be used if you need to do more with the content other than just output it.
The other approach is to put the get_the_content() function into the if statement, avoid creating a variable, then use the_content() to output the content. I think that is better than creating a variable.
Not a huge issue either way but if you want a quick loading script, do not do both. Do not create a variable that holds content prior to the if statement then also use the_content().
We use get_the_content() because it returns the content where as the_content() prints it, no return possible with that function.