That would be a bug and should not be the case. I just tried it and couldn't replicate it. There is no difference between 7-7-2000 and 07-07-2000 in xsd, and neither in the SPARQL query endpoint.
(This doesn't mean we have no duplicates at all in Wikidata - the post actually mentions five discovered duplicates within Queen Elizabeth II's ancestors. But these are entities, not within the datatypes)
Here are the people in Wikidata born on 07-07-2000: https://w.wiki/3wrj
And here the people born on 7-7-2000: https://w.wiki/3wrk
The results are identical.
(This doesn't mean we have no duplicates at all in Wikidata - the post actually mentions five discovered duplicates within Queen Elizabeth II's ancestors. But these are entities, not within the datatypes)