It is a tricky statement, I have seen many people who do not know how to do something then they attempt to do it them selves to save a few dollars and they mess it up even more some times in a BIG way. This is what happens with SEO and Disavows all the time!!! Have seen many companies pretty much WIPE them selves off the SERPs by DIYing it lol.
I think in general this statement is good for wider business schemed ideas it is very true, take a project on and learn how to do it with staff who have the skills.
A good quote from a old Manager director we had a few years ago when we were going for a 10 million dollar pitch and we did not have the staff do it it.
"Worry about winning the pitch first, then we worry about getting the staff later"....
Saw this happen many times over the years.
James.