"Remember" is in fact an independent imperative clause. The subject of an imperative clause is usually omitted, but understood as "you", compare Please sit down; Don't move; Stop!
"Remember" normally requires an object or some other complement, though it has been ellipted in your example.
I don't think it would be unreasonable to drop the comma and treat "it's not true" as a subordinate (dependent) content clause in complement function:
Remember (that) it's not true.
Alternatively, you could treat the two clauses as independent ones and separate them with stronger punctuation such as a colon: