Elijah may have been thinking only of those in active duty. Sequestered prophets are not exactly at their posts of duty, and might be thought of as having been taken out of action.
But, frankly, Elijah was not thinking very straight at that moment. He was depressed. This is often the state of mind that one encounters following a "mountain top" experience and then facing some great or sudden difficulty. God bore patiently with him, understanding the emotional condition of mankind.
James makes a point of this, hinting at Elijah's emotional weakness.
Elias was a man subject to like passions as we are, and he prayed
earnestly that it might not rain: and it rained not on the earth by
the space of three years and six months. (James 5:17, KJV)
This should be of great encouragement to those of us who may experience deep discouragement. God listened to Elijah's prayers, and continued to care for him, even when his faith had failed and he was fleeing from the face of Jezebel.
Then Jezebel sent a messenger unto Elijah, saying, So let the gods do
to me, and more also, if I make not thy life as the life of one of
them by to morrow about this time. (1 Kings 19:2, KJV)
And when he saw that, he arose, and went for his life, and came to
Beersheba, which belongeth to Judah, and left his servant there. (1
Kings 19:3, KJV)
But he himself went a day's journey into the wilderness, and came and
sat down under a juniper tree: and he requested for himself that he
might die; and said, It is enough; now, O LORD, take away my life; for
I am not better than my fathers. (1 Kings 19:4, KJV)
And as he lay and slept under a juniper tree, behold, then an angel
touched him, and said unto him, Arise and eat. (1 Kings 19:5, KJV)
And he looked, and, behold, there was a cake baken on the coals, and a
cruse of water at his head. And he did eat and drink, and laid him
down again. (1 Kings 19:6, KJV)
And the angel of the LORD came again the second time, and touched him,
and said, Arise and eat; because the journey is too great for thee. (1
Kings 19:7, KJV)
And he arose, and did eat and drink, and went in the strength of that
meat forty days and forty nights unto Horeb the mount of God. (1 Kings
So discouraged he had been that he had asked to die! In this state of mind, his perceptions were clouded. Elijah was not deliberately lying; he was simply forgetful of anything positive at that moment.
God's answer, of course, was to correct him.
Yet I have left me seven thousand in Israel, all the knees which have
not bowed unto Baal, and every mouth which hath not kissed him. (1
Kings 19:18, KJV)
The significance of this statement is reinforced in the New Testament when it is recorded again for our instruction.
But what saith the answer of God unto him? I have reserved to myself
seven thousand men, who have not bowed the knee to the image of Baal.
(Romans 11:4, KJV)
Elijah, while not intentionally lying, was forgetful of God's benefits, and, in his great discouragement, his thoughts had become self-centered. God reminds him, and all who are similarly discouraged, that He has others, and that we are not alone in our service to Him.