Trial and error is a terrible way to learn a language - you're likely through "trial" to end up with a great deal of incorrect information in your head. See/hear a new phrase, then associate it with the correct meaning, and you will actually learn and remember the correct meaning. Trial and error for lab science, not languages.
I Google Translated it...sometimes it helps, sometimes it doesn't. It told me this was just "It's ten o'clock" so that's all I entered and it was accepted, but I can see from the translation at the top that it means "sharp" as well.
It is annoying when they throw new concepts in, and I think time is a weird one where you just have to get used to the nuances of how it is written in Spanish.
The literal translation is " They are the ten (hours)...." and that's just the way they express time of day in Spanish instead of "It is....". So every time of day would be expressed the same i.e. " Son las..."/"They are the...(hours)". Except for one o'clock which IS expressed as "It IS (the) one (hour)", i.e. "Es la una", because it is singular
Son las diez means 'it is ten o'clock', it's the way you express time in Spanish and you just have to get used to it.
Once you understand that, you can look at a sentence like this and get the general idea, and then you can probably take a guess at what the rest of it means ;)
Reading the discussions, I'm not sure if "sharp" is really quite the same as "on the dot." If someone is telling you the time, they might say "it's ten o'clock on the dot" or "it's exactly ten," but "sharp" is usually used when telling when something will happen or when to be somewhere. "The train leaves at ten o'clock sharp." The implication being don't be late.
I love trial and error because at least I can apply what I am learning in a given lesson... without even a single obscure question relating to context (time in this case) in the whole accompanying lesson... being forced to stab in the dark at a context robs learners of an opportunity at trial and error.
I think I understand where you are coming from, but from my point of view on my first attempt I "try" to apply my prior knowledge of the phrases "las diez" and "en punto" regardless of the lesson context and work out something that may or may not refer to time or there being ten points (dots). I am then confronted with my "error" of not understanding one or both of the phrases and learn from my mistake that it means "It is ten o'clock sharp." I then can draw the conclusion (even without reading the comments) that "las diez" seems to refer to the time 10:00 and "en punto" is an idiomatic phrase similar to our "sharp" or "on the dot" and if this is a common use of the word "punto" then it makes sense to introduce a [possibly] counterintuitive usage in the lesson where "punto" is taught. [Edit: I can't for the life of me determine which lesson this phrase is in and it has been too long for me to remember... so I'm guessing.]
How is this not trial and error?
Son las diez is a phrase that strongly implies time, since it's the common way of expressing what time it is. It shouldn't need any context to at least trigger that 'is this about the time?' possibility, we should be familiar with it by now.
If that context doesn't jump out at you (and sometimes it doesn't, we're all training our brains to get there after all) then you, personally, need some reinforcement. Getting it wrong and thinking about why and internalising the event helps you to remember. It's not failure, it's a step forward.
Real language doesn't always neatly present context and clues for the listener, your brain automatically does a lot of the work. Duo would be doing you a disservice if it only taught things in isolation - we need the practice, and being told exactly what the context is prevents you from developing the mental agility involved in processing the clues and possibilities. It's a training regime, in a way!