It just didn't make any sense to me earlier. I thought it might have been a typo.

Looking at it with fresh eyes, I realize the intention was to refer to the furnace or the building that houses the furnace.
I agree that we can do without that bit.

On reflection, I think that can be dropped.

@@ -34 +34 @@
-<gloss>(building housing a) traditional Japanese clay furnace for smelting iron (operated with large foot bellows)</gloss>
+<gloss>traditional Japanese clay furnace for smelting iron (operated with large foot bellows)</gloss>

"(building housing a)" ?

Google N-gram Corpus Counts
╭─ーーー─┬─────────┬───────╮
│ 蹈鞴　 │   2,420 │  1.5% │[rK]
│ 踏鞴　 │   1,996 │  1.3% │[rK]
│ たたら │ 120,062 │ 76.1% │
│ タタラ │  33,354 │ 21.1% │add
╰─ーーー─┴─────────┴───────╯
Both kanji forms are jukujikun (smk, meikyo, etc.)

Daijs second sense:
2（「鑪」とも書く）1の装置をした砂鉄精錬炉。 (Their entry uses 鑪 as a search key)

https://ja.wikipedia.org/wiki/たたら製鉄

Wasn't sure what to make of this and sort of forgot.
I don't see the mentioned clay slab meaning for "たたら", but the tatara furnace was built out of clay so that's maybe where that's from?

It seems たたら originally only referred to the furnace, but later came to be used for the building housing said furnace.

@@ -5,0 +6 @@
+<ke_inf>&rK;</ke_inf>
@@ -8,0 +10,5 @@
+<ke_inf>&rK;</ke_inf>
+</k_ele>
+<k_ele>
+<keb>鑪</keb>
+<ke_inf>&sK;</ke_inf>
@@ -11,0 +18,5 @@
+<re_inf>&gikun;</re_inf>
+</r_ele>
+<r_ele>
+<reb>タタラ</reb>
+<re_nokanji/>
@@ -17,0 +29,7 @@
+</sense>
+<sense>
+<pos>&n;</pos>
+<misc>&uk;</misc>
+<s_inf>also written 鑪</s_inf>
+<gloss>(building housing a) traditional Japanese clay furnace for smelting iron (operated with large foot bellows)</gloss>
+<gloss>Tatara furnace</gloss>