I just read your posts Molegrips, it seems your the one who doesn’t have a clue what brake jack is.
Brake jack is when the rear suspension is compressed under braking, making the rear feel rigid and reducing suspension function. Its called brake jack because it gives the bike a jack hammering effect over bumps, its nothing to do with the suspension extending.
Single pivots are not the only suspension design that suffer from it, Four bar is to, hence why the EX I had felt just as bad as the Five.
Chain growth isn’t just a problem with single pivots either, VPP and DW link also suffer.
If you use use selective braking (not braking over rough sections) its not a problem, like I said you can use it to your advantage too.