It is likely interpretation 1, due to the initial HP difference between the two interpretations.

Assuming you go to level 20, here is the range of HP the defender could have under the 2 interpretations, with your Intelligence score assumed to be 18 (+4 modifier).

Interpretation 1: (5x Artificer level) + Intelligence modifier (4) + Iron Defender Constitution modifier (2) = 21 (5x3 + 4 +2) HP at level 3, to 106 (5x20 + 4 + 2) HP by level 20.

Interpretation 2: (5x Artificer level + Intelligence modifier [4] + Iron Defender Constitution modifier [2]) = 45 (5x[3+4+2]) HP at level 3, to 130 (5x[20+4+2]) HP by level 20.

As seen by interpretation 2, its HP would likely be higher than the Artificer themselves at first, and would be equal to an average of 2d10 + 4 HP per level, higher than any class.

It is likely interpretation 1, due to the initial HP difference between the two interpretations.

Assuming you go to level 20, here is the range of HP the defender could have under the 2 interpretations, with your Intelligence score assumed to be 18 (+4 modifier).

Interpretation 1: (5× Artificer level) + Intelligence modifier (4) + Iron Defender Constitution modifier (2) = 21 (5×3 + 4 +2) HP at level 3, to 106 (5×20 + 4 + 2) HP by level 20.

Interpretation 2: (5× Artificer level + Intelligence modifier [4] + Iron Defender Constitution modifier [2]) = 45 (5×[3+4+2]) HP at level 3, to 130 (5×[20+4+2]) HP by level 20.

As seen by interpretation 2, its HP would likely be higher than the Artificer themselves at first, and would be equal to an average of 2d10 + 4 HP per level, higher than any class.