Are "considering that" and "given that" equivalents?

No, they do not mean the same thing.

"Given that aardvarks have spleens, they will resist infection by encapsulated bacteria" means that you are assuming that aardvarks have spleens and are explaining the consequences of that assumption. You are not making any statement as to whether aardvarks actually have spleens, but you are acknowledging that whether aardvarks have spleens affects your claim and you are identifying your assumption.

"Considering that aardvarks have spleens, they are resistant to infection by encapsulated bacteria" means that you know aardvarks have spleens and are explaining the consequences of that fact. You are stating that there is reason to believe that aardvarks have spleens and that you are basing further claims on that reasoning.

Considering and given that have almost the same meaning and can interchangeably act as a conjunction.

considering conjunction & preposition

Taking into consideration.

(as preposition) ‘considering the circumstances, Simon was remarkably phlegmatic’

( as conjunction) ‘considering that he was the youngest on the field he played well’

given that (conjunction):

when you consider something

It was surprising the government was re-elected, given that they had raised taxes so much.

It seems that both are equivalent for all practical purposes, in the setting you provide.

One could possibly come up with examples where they differ, if they were not used at the beginning of your examples, thereby limiting the scope to the content of that sentence only. Limiting consideration to the two alternate examples that you do provide, no uncontrived candidate ABC comes to mind that would differentiate those two sentences to any significant degree.