It sounds like that salesman was one of those all-or-nothing, black-and-white thinkers.
I agree! The looks I get when I mention dirt in an aquarium reminds me of this.
Welcome to planted tank!
are some interesting low tech tanks.
many are doing the El Natual method that Diane talks about.
Homer_Simpson did an experiment comparing the El Natural method to the water column dosing method.
I am doing the El Natural
method. Started with it for read people only do water changes 1x a year and don't add ferts. Well after 3 months BBA (black brush algae) suffocated my plants. Now I do water changes monthly. Did not add the coral as recommended for have high ph and it raises the ph. Used pool filter sand from Lowe's to cap the dirt. For dirt I like Scotts top soil. I dose minimum for have a version of mineralized soil. Have a very thin layer of laterite, thin layer of reptile coconut bark, top soil approx 1 in. and sand approx 1 in. Total height of substrate is 3 in.
Some are simply afraid of using soil in an aquarium. At the worst you get a sand storm after moving a lot of plants around. It won't hurt the fish unless there is ammonia in the substrate, which occurs when the substrate is very old or you put house plant ferts in it. In fact, when I used play sand I had a bad sand storm in the tank and it didn't affect the fish.
When I move I going to try Hydroton clay (hydroponic supply) under the substrate. It is used for lily's.
Keep in mind that no ecosystem is the same. In fact one told me he had 3 10 gallon tanks side by side. He dosed them all the same and had 1 light across the 3. With one he had a problems with algae.