Pardon me, but that's a proven fact how? You can hardly deny that the DP poster sure made it look like Brock would catch Hikozaru. If it was only on there to make sure all the starters were advertised, why put it specifically next to Brock, especially when Pochama and Naetle were clearly matched up with Hikari and Ash respectively? That's hellishly misleading if he's not going to catch it. Not to say that a DP poster would never mislead us (haha), but still.
And before somebody makes the predictable counter argument, I wouldn't consider it an impossibility for Brock and Shinji to both have one of the same Pokemon, even if it might not be likely. Anyone ever consider that giving Hikozaru to two main characters would be a good way to help bridge the gap in its screentime compared to Naetle and Pochama? If only Brock had caught it, we'd have ended up with a repeat of the Hoenn situation, with Mudkip being shafted in favour of Treecko and Torchic. And if only Shinji has it, it won't be much better off; he doesn't appear all that frequently.
It's not even true that no two main characters have ever had the same Pokemon at the same time before - in Johto, Ash and Casey both had Chikorita. Casey might not have been as prominent as Shinji, but she was at least showcased more than Nicolai. And Chikorita/Bayleef/Meganium was her signature Pokemon, so it always got an appearance any time she did. And although May's Bulbasaur and Squirtle weren't on her team until after Ash's had been Oaked/sent with Jenny, they were still redundant advertising in many ways.