I use soap every wash on well-seasoned cast iron too, no problem at all. You need to make sure to wash all the soap off, and dry it well -- or at least I believe it's important to do that, I guess since I always do it, I could be making assumptions too, maybe it's just fine if you don't?!

But yeah, the idea that you have to leave the cooking grease and fat on cast iron and aren't allowed to use soap to cut the grease and get it off.... is an odd one.

You are supposed to take your greasy cast iron pan, and put it in the blast furnace it the backyard to clean it!

Maybe I've been playing a bit too much GTNH.

My grandmother used to literally build a fire in the back yard and throw the skillets in whenever she thought they needed it (maybe every few years).

This was long, long before Alton Brown was a thing.

Best way to dry it is to throw back in the stove on high for a few minutes.