Chapter 4 should be .bashrc instead of .bash_profile

[hisacro]

I guess you mistook .bashrc for .bash_profile

"The ~/.bash_profile is a list of Unix commands that are run every time we open our terminal,"
.bashrc is being read by non-login shells and alias are usually defined in .bashrc

source

[matt-kita]

https://www.gnu.org/software/bash/manual/html_node/Bash-Startup-Files.html

Just because something is "usually" there, doesn't necessarily mean it "always" has to be there.
It really depends on what you wish to achieve. And if you're currently running a login or a non-login interactive shell.
Question is: "Does it really matter (for non-experienced beginner)?"

[hisacro]

Just because something is "usually" there, doesn't necessarily mean it "always" has to be there.

sourcing .bashrc becomes pointless and every time you need to logout to invoke changes in bash config.

being a beginner doesn't mean you have to unlearn things.
it's defined fairly simple in bash man page

[matt-kita]

sourcing .bashrc becomes pointless and every time you need to logout to invoke changes in bash config.

You don't need to logout every time to invoke changes in bash config. It's also defined fairly simple in bash man page how to execute bash subshell as a login shell (or at least make it pretend to be). As is explained how to run bash login shell without any configuration (e.g. --noprofile --norc). Where is it stated that you have to unlearn things? Not by me. Looks like this discussion becomes as pointless as sourcing .bashrc :D

[archqua]

Well, I'd agree that this part is not the best it could be. As far as I understand, adding source ~/.bash_profile to ~/.bashrc might be somewhat reasonable substitution for defining aliases in ~/.bashrc and it's worth mentioning. I was confused by this at first and it's not nice to confuse newbies (like me).