It's really simple. 20% may be good compared to some companies that sell total trash,. but don't fool yourself, getting 20% less than what you paid for is not OK. You may be fine with it and if so, then that is your decision to make...but pretending that it is OK for a company to moderately under-dose a product simply because some companies are worse, is like saying it is OK if someone steals $100 from you because everyone else stole $1,000. It's flawed logic. Just because some companies are worse,it does not mean that one should lower their standards and accept mediocrity.
Now, as far as I am concerned, this very well may have been an accident (it probably was because under-dosing S23 by 20% would've made basically no difference in terms of profit, considering how inexpensive S23 is to manufacture), but anyone who finds themselves in this situation has a right to contact the company and expect either a partial refund, above average future discount code, or even a whole new bottle of S23.
In other words, it's nothing to crucify the company over, but it shouldn't be ignored, either. If something was wrong with one of my products, I would WANT the person to contact me and let me know, so that I could make it right and hopefully maintain their business. It's far better to make a little less on one sale than to lose a customer permanently.