You're right, in a vacuum that line could mean almost anything. But given context, it can't. For starters, we already knew about 5 Ball Strike and 10 Ball Strike - we didn't know it'd be possible to actually split a single ball, but we knew Oni could hit this many back in chapter 20. Hence this just doesn't make sense to foreshadow. Then there's the distinction between a 2-string BJK and a full-string one, but do you seriously think someone on the 1st String has never seen the latter when Oni even used it against a random MSer, 3rd court's reserve player and even random low court players know about it? I can't disprove this, but it strikes me as highly unlikely.
Finally, there's AnS - the problem with AnS is that it wasn't actually introduced until Tokugawa used it, hence it again doesn't make sense to foreshadow this for Oni's match. Kijin is the only realistic choice left.
Next, chances are that Byoudouin actually wouldn't have lost to AnS!Tokugawa even without Pirate. As we've established, Oni didn't know about Byoudouin unlocking the aura before, but
he still urges Tokugawa to finish the set immediately once Tokugawa gets his first set point. After thinking more about it, I also reject the notion that Byoudouin literally played at H1 level. In my eyes he was holding back in the sense that he didn't use any special moves besides that one Destruction at the beginning, but he was still rallying at his regular H3 level - Oni was most likely concerned that Byoudouin would start pulling out World Techniques at that point.
Byoudouin setting up the stage to match what happened in his match with Oni is valid, but I don't think this necessarily includes him also playing at that same level, just that he and Tokugawa would trade games the same way and he'd return a Destruction the same way. After all, if Byoudouin just played at H1 level and Tokugawa was above that, the setup wouldn't work at all.
Lastly, how does Tanegashima being No. 2 to just Byoudouin not make sense? Even under your assumptions, Byoudouin was likely stronger than Oni by the end of H1 (and he definitely is now during H3), so if Tanegashima can draw against Oni he loses to Byoudouin.