If the wording had been "when you expend a spell slot", you might have had a case. But it's "by expending a spell slot."
Summoning the turret takes an action and the use of your smith's tools. You can do this for free once per long rest, or more often by expending spell slots. This is clear as glassteel. If it was intended that just using the spell slots recovered the ability, the wording would be similar to that of the Wild Magic Sorcerer's Tides of Chaos, which reads:
Starting at 1st level, you can manipulate the forces of chance and chaos to gain advantage on one attack roll, ability check, or saving throw. Once you do so, you must finish a long rest before you can use this feature again.
Any time before you regain the use of this feature, the DM can have you roll on the Wild Magic Surge table immediately after you cast a sorcerer spell of 1st level or higher. You then regain the use of this feature.
The equivalent for the turret would be: "Once you use this feature, you can’t use it again until you finish a long rest or expend a spell slot for any reason."
