I've used hardware store lye (Red Devil, Roebic), Lye Guy lye, and Essential Depot lye. They all work fine.
What "100% NaOH" means is there are no other chemicals ~intentionally~ added to the product. Some drain cleaners have other ingredients in them other than NaOH to help the cleaning action. Obviously those products aren't good for soap making.
What "100% NaOH" does not mean is the product is 100% pure NaOH with absolutely nothing else in the product. There will always be some amounts of impurities in the NaOH, due to the manufacturing process and due to the fact that NaOH reacts very quickly with carbon dioxide and water vapor in the air.
Tech grade and food grade NaOH come out of the exact same manufaturing process. The main difference between the two types is the additional lab tests done to document the amounts of impurities so the NaOH can be labeled and sold as suitable for human consumption.
Other than less stringent testing, tech grade NaOH is not any different chemically than food grade NaOH and is just as safe and just as suitable for soap.
So if a person wants to use food grade NaOH for soap making, by all means use it -- no problems, no worries. But if a person has been using hardware store NaOH or other tech grade NaOH, they should not lose any sleep that their soap is somehow inferior. It isn't.