@oristarA,
The author has confused the word 'credulity' with 'credibility'
A person can be gullible, too ready and willing to believe something - a credulous person
an explanation can be credible - convincing, able to be believed
But an
explanation cannot be gullible.
'strain the credibility of that explanation' :
Is there "junk DNA", carried over and along as life evolved? Is it that nature hasn't emptied her rubbish bin? Is that the explanation for this DNA?
OR
Does it still "play important regulatory roles?" Well, when we look for proof , we find examples that make it hard to believe that explanation - it doesn't seem credible. It strains our capacity to believe that that is the real explanation.
It strains our credibility, our capacity to believe it.