I bought an Extang BlackMax Tonneau Cover on my 98 S-10 probably around 2000. The first cover lasted until around 2016 when I replaced it. Ill be honest, the replacement was a perfect fit, but isnt lasting as well...
So I have an old school snap cover. Ive used my truck for all kinds of stuff, from luggage to tools to wood to furniture, and a ton of other things.
A snap-on cover with a soft top is easy to get off, and hard to get back on, especially in the cold. But you can unsnap it to allow just the section you want to be open. The loose section has to be prevented from flopping around, but its very flexible. Any sort of hard cover, be it a full length or a 1/3 section type offers less flexibility, especially the full length type. It looks to me like the one you linked to has a metal frame and a soft cover attached to it.
I wouldnt be considering any soft type cover to be "secure". Have I ever had any issues? No. But the risk is always there. Ive seen hard ones (including roll-up hard ones) which have locks, and a truck with a locking tailgate and locking cover might be as secure as, say, a soft top jeep. Someone would have to really want to get in, and need to cut their way in, to see what you have. If you want security, you need a harder cover, and the downsides that come with it.
Id think of a tonneau as more of an easily employed dry storage. I can confidently say that the cover on my S-10 has never gotten wet inside from rains or storms.
An option Ive seen that I like, if youre concerned with storage, is to get a metal locking tool box, and then a shorter tonneau that offers the dry storage.