Already discussed on the previous page. You'll note the last line of the quoted paragraph, that Disruption can overcome Steadfast, is completely wrong, and directly refuted in the Steadfast section. GW often gets the rules wrong in it's tactica articles and Battle reports, that's why no-one I know would ever accept them as rulings of any sort.
That said, it's still a case of "RAI, they should, RAW, they don't", so against any reasonable opponent, you shouldn't have a problem. Just be sure to check in pickup games and Tournaments, because lack of it can completely change the game for Bretonnians.
Again this is wrong on your part, but only because you are meshing two ideas together.
Disruption negates the
combat resolution rank bonus of an enemy unit, provided you are hitting them in the flank/rear with sufficient numbers (in the lance case, needing only 6). The quote I posted follows with this logic.
Steadfast can be negated by having more ranks. Brets Lance ranks up easier (only needing to be 3 wide). 12 brets knights in lance formation is 4 ranks, which is more than a lot of the infantry blocks out there. Page 54's diagram elaborates on the idea that one can be steadfast while being flanked, so there is no disputing that idea. The last sentence I quoted is GW continuing on the thought of bret formations being 3 wide, it is easy for them to rank up and compete against / beat infantry blocks. Please note they are seperate thoughts (/sentences) with no conjunction used.
Can lance formation disrupt? Yes.
Can lance formation break steadfast? Provided the sufficient ranks, yes.
Does disrupting a unit prevent steadfast? No.
Is it game breaking? No.
Is anyone going 'OMFG Brets are Tier 1!'? I haven't seen it.
Is it rules-lawyering to try and even argue this, despite common sense? Yes.
Do I think the whiners need to go back to warhammer 40Kindergarden? Yes.