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.