The stupid thing about this advice is the 'date' part. You should never date any person you're not attracted to.
I can't help having some sort of 'flirt tone' in my voice whenever I speak with women, basically all women. Young, old, attractive, unattractive. I didn't even notice this, it was brought to my attention by my wife, who said she could by the tone in my voice whether I was speaking with a man or a woman on the telephone.
Being friendly and flirty people you're not attracted to costs you nothing but is very educational.
Flirting is about making the other person feel better about themselves. If you can make someone feel good about themselves, they will remember that feeling as a result of being in your company. When people feel good in your company, you will attract more and more people.
I noticed about flirting with people I was not attracted to is that it's actually more difficult to successfully flirt with unattractive people, especially the unattractive people with low self-esteem, because they don't trust your sincerity.
Making someone who feels good about themselves feel better about themselves is a lot easier than making someone who feels bad about themselves feel good about themselves. Flirting with people who know they are attractive is easy in comparison, because your compliments confirm their attractiveness, they have no reason to doubt your sincerity. The difficulty convincing a suspicious person of your sincerity will make you work on your skills of persuasion and how to increase your congruity.
This is also how I learned how to flirt without an overt sexual undertone. Basically, it's like paying men a compliment without having to add 'nohomo' to your statement. If you can make an unattractive person (who is possibly attracted to you) feel good about themselves without sexual undertones that send the wrong signal, it's a deceptively subtle manipulation that you can often use to create goodwill in those you loathe.