"Available" is rather a vague term that can be interpreted in many different ways. The message could be improved by saying "Memory available to mongod process is less than total system memory" or something like this.

Example od the current message:

{"t":{"$date":"2021-01-25T09:32:52.556+00:00"},"s":"W",  "c":"CONTROL",  "id":20720,   "ctx":"initandlisten","msg":"Available memory is less than system memory","attr":{"availableMemSizeMB":5120,"systemMemSizeMB":32011}}