DF1 and DF2 neither mention what is the "default" TL (unless I've missed it). Of course this should be obvious that it is TL3 right? Well, not exactly. Pretty much all TL4 armor, melee weapons, and gear are available for use in DF.
It is also worth mentioning that DF2 states: "Assume that dungeon fantasy is TL3 for the purposes of first aid." But what should we assume it is the rest of the time?
In DF3 the Wildman template lists Low TL 3, and in the description limits them to TL0 gear. This implies that DF is in fact TL3.
Now, why does this matter? Well first off, the Barbarian template lists Low TL, so I'd like to know it means to be Low TL 1 vs, Low TL 2. Second, TL says a lot of what isn't said about the world. It's short-hand for all the things that it would wasteful to list. And lastly, DF as a genre has often borrowed from Sci-Fantasy. Dying Earth (series and sub-genre) was (and is) a major influence. So, there is a chance that the players might find a "Strange device of unknown magic!" that is some sort of gizmo, and need significant TL penalties to attempt to use or understand.
For my game, I'm going with TL3 with some bits of TL4 blending in (but treated as TL3 for game purposed, ie no penalty to use TL4 gear). Of course, who knows what strange things the dwarves have dug up? Or what vile techno-magic is used by some Elder-Things?