James Gasik
We don't talk about Pun-Pun
You are correct that it's a 3 in 10, derp, but how the surprised modifiers in the DMG apply to Thieves is a bit odd. They are listed for "silenced", "invisible", "camouflaged", "poor light", "darkness"Oh the surprise chance could be a little better than 3 in 6. For one thing, surprise in 2e was 1-3 on a d10. Then, if the thief succeeded in their stealthy skills (which they could better optimize), they could improve that another 4 points for 6 in 10.
Hide in Shadows is not invisibility, and doesn't even function in darkness- camouflaged maybe? And I'm not sure that "Move Silently" is meant to be the same as magically silenced, but that might be splitting hairs- I'm used to AD&D DM's who rule very conservatively.
But you're right, the chance to be surprised could be better. Elves and Halflings impose a hefty penalty, with some caveats.