It wasn't Tesla. His technical "wireless wizardry" is much exggerated by his groupies today. In reality he was years behind Marconi (and far less sophisticated technically), and in 1903 he simply wasn't up to this. (And outside the US, the US Supreme Court decision on Marconi's patents is widely regarded as pure jingoism -- and contrary to the usual blurbs of the Teslaphiles, it was not based solely on Tesla's contributions.)
This "hack" was really a brute force attack presented in a tricksy manner, and was only possible because a major commercial competitor was behind it, and put a lot of money into it.
The first demonstrations of radio transmission were made by Heinrich Hertz in the 1880s. In the 1890s, a lot of people had been working on radio phenomema: Oliver Heaviside, Edouard Branly, John Stone, Oliver Lodge, Alexander Popov, Nikola Tesla and of course Guglielmo Marconi -- and no doubt many others. Most of them were concerned with it purely as an interesting new phenomenon. Tesla was primarily interested in it for energy transmission (disastrously for him, as such a thing is possible in principle, but far too grossly inefficient for practical use -- and Tesla should have realised this if his mathematical theory had been stronger.)
Marconi was interested primarily in building a practical communication system, and his contribution lay largely in improvements in antenna design that were a result of his own experimental work; in bringing together the incremental improvements of several others (some of whom where paid for their work); and in what we would today call "optimisation." These were not minor contributions; they turned radio transmission from a laboratory toy with a practical range of a few yards, into a practical communication system with a range of miles. (He reached 10 miles by 1897, 60 miles by 1899, and thousands of miles by 1901; no-one else was even close to this.) For example, Marconi's coherers were certainly based on Branly's work, but Marconi's were more than an order of magnitude more sensitive.
Part of this was the use of "syntonic" circuits which were tuned to particular frequencies -- and subject to numerous patent disputes, as to who invented them. Probably Lodge was first, although certainly the idea is strongly suggested by Maxwell's basic theory from the 1860s, and was independently re-invented by several people in rapid succession around this time. A tuned circuit enables the receiver to have far better discrimination of signal over noise, and so to pick up much weaker (i.e. more distant) signals. This was what enabled Marconi's equipment to attain transatlantic range.
So to cut a long story short ("too late!" they cried), Maskelyne's crude spark transmitter could intrude on the demonstration at the Royal Institute, but only if it were very much closer than the intended transmitter. The fact the signal was picked up even by the arc lamp suggests that it was less than a hundred yards away. Similarly, Maskelyne's very much cruder receiver could pick up Marconi's transmissions from Cornwall because it was hundreds of times closer than the intended recipients. However it certainly could not receive the replies from ships at sea, and unless the message itself gacve it away, it could not tell for whom the transmission was intended. Even though Maskelyne was supposed to be demonstrating that wired telepgraphy was more secure, his demonstration actually offerred less in the way of a "hack" than simply tapping a telegraph line!
Maskelyne's demonstration did show that "syntonic circuits" offer very imperfect security, yet Fleming was right to suggest that it was more of a trick than a hack.