This is not necessarily true....ADC to DVI adaptors exist, and DVI to VGA adaptors exist, and I have gotten VGA LCD panels running fine on my IIsi with RBV video, including my old Dell 24" 2407WFP. And yes thats a crap lode of adaptors, but it works. You need the following to make this happen:

ADC to DVI adaptor (not sure who makes em)

DVI to VGA adaptor (free with every video card!)

VGA to DB15 Apple (Griffin makes these among others)

...and lastly, the special IIsi/IIci Synch on green adaptor! Check

online or I may have extras...

Attach all these together and BAM there u go. Albeit at 640x480 unless you have a NUBUS video card in your IIsi.