What's the difference between the terms "muscle" and "muscle organ"?

The word "muscle" is applied to both the organ and to the tissue.

• There are 3 types of muscle tissue: skeletal, smooth, and cardiac.

Below is a decent quote from SEER explaining this (my emphasis added):

A whole skeletal muscle is considered an organ of the muscular system. Each organ or muscle consists of skeletal muscle tissue, connective tissue, nerve tissue, and blood or vascular tissue

If the word "muscle" is used on its own (i.e., without specifying "muscle tissue" or "skeletal muscle"), it is typically referring to the whole skeletal muscle organ.

• Examples: chest muscles (pectoralis muscles), thigh muscles (quadriceps and hamstrings), etc.

---

Note: the word "bone" is used in a very similar manner.

• "Bone" can refer to the entire organ (e.g., femur) or to one of the two recognized bone tissue types (compact bone and spongy bone).

In the context you state, seemingly nothing. "Muscle" is listed as a homonym of "Muscle organ". So it seems, for the sake of the website you like to, there is no difference (unless they are using homonym differently).