Well done. You say you've not ridden a bike since you were 12, but if you don't mind me asking, how old are you now? If it's been a long time, it will take some practice to get back into the swing of it, but the first step is the hardest.
It's true that £149 will go a lot further at Decathlon than at some other places, but it's still a very cheap bike, and you'd expect it to have quite a few limitations. I really don't like the look of that saddle, for example, and the brakes and brake levers are probably made of cheese. That said, I applaud sensible decisions like the single chainring - keeps the gearing simpler for new cyclists (ie the kind of people this bike is aimed at), and fewer components (no front derailleur, no front shifter) means the budget isn't spread quite as thinly - or means more money to spend on other components, like the quick-release hubs, which are arguably more of a benefit than extra gears.
I can't comment on the specific tyres the bike comes with but I've never seen tyres on a bike that price that I'd want to actually ride on.
It's badly expressed, sure, but I take that to mean something along the lines of "If you're the kind of rider who doesn't expect to be out on their bike for more than two hours at a time, this bike will be just fine for you". I'm sure the bike itself would be happy to keep going all day, but two hours on that saddle sounds like more than enough punishment to me. In which time, you probably won't have covered much distance because those tyres are likely to be very sluggish. That's not necessarily a problem if you're just pottering about in the park or along the canal towpath, or riding a couple of miles to the shops, but if you're interested in longer rides, you probably want something that's going to be less of a chore to ride.
For someone getting back into cycling after a long break, it's probably fine, but the OP may find they want something a bit livelier before too long.