Are certain atheists absolutely sure by saying that there is no absolute truth?

Absolutely

I've never heard any atheists saying there is no absolute truth. You'd have to ask them, if you can find any.

Personally, I'm an atheist who believes in absolute truth. For example, young-earth creationists calculate from biblical genealogies that our planet is 6,000 to 10,000 years old. Scientists conclude from physical evidence that our planet is around 4.5 billion years old. But the truth of the matter is absolute. There's an age that the earth actually is in reality, independent of how old anybody thinks it is.

"...by saying..." adds nothing but confusion and the implication that your question is somehow incomplete. If you are asking what I think you are, my answer is no. There actually are a limited number of absolute truths.

Personally, I am certain the scientific laws used to describe the behavior of objective physical reality are as close to absolute truth as we'll ever know. Pretending ancient Roman propaganda is the Word of God is foolish superstitious nonsense.

If there is no absolute source of morality...

then....

a person may as well get what they can get, in any way they can, any way they can get away with.

Say, like Donald Trump. But, also like you and me and everyone else, in youth, at some point, in smaller ways.

It's when a person becomes not just occasionally good but more noticeably good, or does the right thing when it is much harder to do, that we all take notice.

There is Someone that can change us profoundly. Seek Him, in the pages of the gospels.

The only way primitive religion exists today is through the child abuse of forcing it into very, very young children but thanks to better education and growing intellects so many teens are able to discover the truth, throw off the indoctrination and step into the real world!

Atheism is not a conscious decision or a belief but a realisation, those that cannot escape remain prisoners of their conditioning!

The first person to produce a single tiny little piece of verifiable evidence for any god will become world famous and mega rich!

Academia states that in the absence of any sort of evidence of the existence of something it must be deemed not to exist until verifiable evidence is found - thus god is held not to exist pending some sort of verifiable evidence.

Only religion claims absolute anything which is why it takes a back seat to secular law and common sense.

Who exactly said there were no absolute truths? Are you confusing truth and morality?

The claim that "there is no absolute truth" is a claim about absolute truth. It is self-contradictory.

absolute truth only exists in mathematics

There is no such thing as absolute truth. Too many shades of grey on everything out there.