If you spend 10,000 hours divided evenly over a period of 10 years, then you’re only spending 2 hours and 45 minutes a day practicing your skill of choice. If you haven’t got a day job, then there’s lots of room in your day for getting good at multiple things at once.

Let’s crunch the numbers:
1 year has 8,760 hours.
10 years have 87,808 hours (including two leap years).

Of those 87,808 hours, let’s assume the average person will 30% of them asleep (that’s between 6 and 8 hours a night). That leaves 61,465.6 hours. Let’s get rid of the remainder, for convenience’s sake.

So now we’ve got 61,465 hours of discretionary time. Holmes went to school, obviously, and this is likely where he learned some of the rudimentary skills that he later used to fight crime.

Someone correct me if I’m wrong, but Holmes never had a day job outside of being the World’s First Consulting Detective, right? That leaves a ton of time for him to practice the varying skills that he knows will be useful to his pursuit of crimefighting.

These numbers, furthermore, assume that Holmes began his work at age 17, and made no progress of any sort beforehand.

In short, I don’t find Holmes’ accomplishments to be superhuman. Extraordinary, sure, but superhuman, no.