Nah, it’s because the taste sticks and you shouldn’t scrub the layer of burned in away. The longer a cast-iron is in use the more of a unique taste it develops by itself, you can taste which cast-iron was used for cooking when you know them.
I have three and can taste the difference.
And no, its not dirty, its the intended use of cast-iron.
Seasoning is not about taste. You’re just using a dirty pan if it has stuff burned onto it. Your pan should not be imparting any taste into your food.
Step 4 literally says to scrub until all of the brown bits are gone. Look at the picture in step 5, does that look like anything is burnt onto it? I see nothing in that says anything about taste.
Also, wikiHow is a garbage source as anyone can submit anything they want. They don’t have to have any clue what they’re talking about.
Here is cooks illustrated, a well respected cooking magazine that tests everything, talking cleaning.
And yes, they say soap is fine. If mild dish detergent is removing your seasoning, it wasn’t seasoned properly.
So are toenail clippings I find on the street if I heat them up first, doesn’t mean I want to put them near my food.