Hi there! I'm glad you're having fun with the game!
You're interpreting it right - in that specific situation, Line of Sight would be blocked. Now, while a corner of a square containing a piece of terrain or a Unit blocking LoS may seem unrealistic, the game leans heavily into abstraction, including abstraction of exact terrain object and Unit positions within the squares.
Archers/Slingers can still shoot without Line of Sight, albeit with a Disadvantage, so they can still provide supporting fire from the back ranks. That being said, following the standard historical order of battle is the best way to use ranged troops in the game as well - skirmishers in the first rows would, well, skirmish before the main infantry lines clash, at which point the skirmishers would withdraw behind friendly lines and stay in reserve, try to find flanking positions, or fire over their lines if able.
Ultimatelly, in the era, ranged troops were rarely, if ever, the decisive winning factor of the battle, and if they were - it was due to superior positioning (ex. Battle of Agincourt). In the game, think of ranged troops as a great way to weaken the enemy before infantry lines clash, and try to increase their utility by superior maneuvering and taking high ground, like you noted.
Let me know if you have any other questions!











