I have not heard this before. Abul Bakr (ra) was was not the only true caliph, there is no evidence in the sunnah or serrah to support this.
And the Prophet (saw) said: "The Prophets used to rule Bani Israel. Whenever a prophet died another prophet succeeded him, but there will be no prophets after me; instead there will be Khulafaa' (Khalifahs) and they will number many". They asked: what then do you order us? He said: "fulfil allegiance to them one after the other. Give them their dues. Verily Allah will ask them about what he entrusted them with.
The Prophet (saw) said "The Prophethood will remain among you for as long as Allah wills, then Allah will lift it when He wishes to, then it will be a Khilafah Rashidah (Rightly Guided) on the method of the Prophethood, it will remain for as long as Allah wills, then Allah will lift it if He wishes, then it will be a hereditary leadership which will remain for as long as Allah wills, then He will lift it if he wishes, then it will be a tyrannical rule, and it will remain so for as long as Allah wills, then He will lift it if He wishes, then it will be a Khilafah on the method of the Prophethood, then he kept silent."