The subject is a little misleading since you’re question only asks about one specific function. I think the general consensus around your question is that yes it can, but it won’t be nearly as easy. You can start here:
SmartThings is primarily a home automation hub. You can disarm/arm with presence, door locks, your phone, community supported keypads, etc. There are many threads on here discussing the merits of using ST as a security system.
Your keypad is typically connected to an alarm controller box which manages the functions of the keypad and alarm (lights, siren); I assume you have one of those somewhere. These can be controlled by ST in some cases. Most of us use a DSC system and there are many threads here describing the implementation of Alarmserver on PC’s and Raspbery Pi’s. Other controllers have also been experimented with and implemented.
I am using it for home security. So the ad is correct.
It is not necessarily always as easy as ADT, but then again you also don’t have monthly fees.
The process I have configured uses an inexpensive android tablet plus presence sensors plus a smart lock. The presence sensor disarms the system when it arrives, so in that regard it’s even easier than entering a code. But entering the correct code on the smart lock also disarms the system while unlocking the door… again, that’s even easier than ADT. I also have the ability to enter a code on the android tablet to disarm the system, which is just as easy as ADT. Oh, and you need not press a button to arm the system when you leave; when X condition is met (such as all presence sensors have left), it arms itself automatically. SO EASY!!
But what is ST for? Aside from what I just mentioned, it does all sorts of things for me. It alerts me when the tub is full. It turns on lights when we arrive home; it turns off lights in rooms/closets not being used. It changes intensities of various lights based on conditions. It can turn on my stereo and tv from web program notifications. It interfaces my devices to Alexa, so I can voice control my home. It opens and closes my garage. It captures video when certain conditions are met. It can send texts, and make phone calls, automatically when certain conditions exist.
So I’m not quite certain of your intent in the framing of your exclamations and questions, but be assured it can do the job of home security that you seek. Just be aware that you will have to do some of the lifting.