It might be mathematically questionable but there is no semantic issue with this sentence.
The risk is simply a certainty.
“C’est sûr” has nothing to do with “risquer de” in particular.
“Risquer de” can mean a strong probability, not always negative.
“Ce concert risque d’être une tuerie”
Note that using it positively is a little informal.
“C’est sûr”, in that case, simply means that you agree with a previous statement, often adding information.
“Ce concert va être dingue” “C’est sûr, avec ce groupe c’est toujours génial”
There is nothing wrong with using those two together.
nothing more than others
Although all existing answers are right, saying that this sentence is semantically correct, as native I feel like it is not a sentence I would use.
Since you are asking about usage, I would have probably been annoyed by this potential contradiction.
what I would have use
C’est sûr que les garçons risquent de s’y sentir les bienvenus.
I would prefer to explain the guarantee by using:
C’est sûr que les garçons s’y sentiront les bienvenus.
Or, if you rather want to underline the chance it is not sure:
Il y a un risque que les garçons s’y sentiront les bienvenus.
I’m curious, did you find this sentence in a book or have you written it?