Exactly. The Independance exploit, took advantage of a loophole in the PS1DRV module from the ps2 bios. Newer versions had an updated module which prevented the expoit. So, since it was already fixed on the ps2, it wasn´t very likely it would work on the ps3.
Well actually the Independence exploit is impossible because the PS1DRV was removed from the ps2 bios on the ps3. Your also right about all the other exploits being secured.
It is possible that there could be some new exploit since we´re dealing with a different ps2bios ps2mode will run unsigned code. But then there´s always the problem of getting into ps2mode which requires a ps2 disc
Although the mc´s are emulated, if the exploit still worked, it probably would have still worked. at least for ps3s with the EE chip. The exploit was put on the mc but it wasn´t the mc that was being exploited