The headline is good advice but being your own first customer is rarely good.
First the bad:
1: if you have problem X and have the in house skills to fix it, it’s likely that your potential customers can fix it for themselves too.
2: you are not the same as anyone else: you can fake yourself out by believing that your potential customer thinks the same way you do. If your prospects are quite different from you (say you’re a programmer writing a tool for doctors) you’ll work harder to understand their real and perceived needs.
But sometimes this is good advice:
1 - Even if you are different from your customer (say you’re writing something broadly applicable like a web site design tool for small business), “eating your own dog food” will find bugs and infelicities faster.
2 - if your customer has the same needs as you perhaps you already know how to meet said customer easily, such as trade groups you’re already in.
In general the advice in this post was pretty basic and generic but I didn’t want to let the issue above pass I commented.