Not sure who told you that...your factory warranty does not "remain intact" with Dinan mods, as it actually did...say...10 or 15 years ago.
Like with any tune/mod, your factory warranty cannot be voided. A dealer can only deny a claim on a case by case basis, citing yours mods as a (potential) cause, including Dinan mods.
The BMW/Dinan relationship has been over for some time now.
Currently, Dinan claims only to match BMW's warranty coverage. However, you can read many instances of them attempting to deny claims, just as BMW sometimes does. Dare I say getting BMW to cover repairs is easier than getting Dinan to, understandably so.
https://www.dinancars.com/warranty/
But make no mistake, your factory warranty is no safer with a Dinan tune vs. any other tune.
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'02 S54 M3 (500/500 GC/Koni)
'08 N54 135 (JB4, DCI, BMW PS/Bilstein B6s, H&R M3 FSB, Strongflex FCABs)
'14 N55 X1 (JB4, BMS DP, BMS Intake, Alpina TCU reflash, H&R Sports, Bilstein B6s, E93 M3 RSB, Strongflex FCABs, baby seat)
'08 N54 535xi touring (Bilstein B6s, Downpipes, MHD tune, baby seat)