The actual reasoning is FMCB uses a fault on PS1 memory card retrocompatiblity layer for PS2. Once you get FMCB setted up there is no way to reliably run PS1 games BUT! If you have a working laser and get the FMCB memory card out you can use a PS1 exploit (tonyhax is a great example) to run PS1 games from there at full compatibilty, no modchip.
I think tonyhax doesn't work on later PS2, it's not compatible with mine.
I did initially try to use POPS, which is terrible for castlevania SOTN (tried beta 17, cheats.txt, $afemode, compat 1/2, the whole nine yards). crashes all the time. wouldn't trust POPS as far as I could code it myself)
then I bought a real PAL PS1 game and disabled autoboot, and used launchelf to stop the disk and a slide card with a burned CD-R, then launch PS2DISK from MISC. This works for playing PAL games at 50hz, totally (but you can't force 60hz 240p with GSM with SOTN or else you have half the screen blank)
honestly? I never imagined it was so much trouble to do a simple thing as run a PS1 game on a PS2. but at least now I can run SOTN on my PS2, natively, albeit in 50hz mode on my PVM CRT. oh, but I can't save my game, it turns out you need an original PS1 memory card for that, naturally
if any european is reading this, and is starting a fresh, and just wanna play PS1 in 60hz 240p? let me save you a lot of time....make damn sure you get either original american PS1 and use tonyhax, or get american PS2 (SCPH-7500X and above) and use mechapwn with DEX
edit- after trying out the mechapwn DEX option, it seems the PAL 50003 PS2 does work with PS1 backups, no real PS1 game needed as a swap-out and no slide card (manual sneaky removal of the disk) needed to boot the game.
I wonder if I can now switch the region of my console with the CEX option, then install the DEX option for that region afterwards, in order to play the USA region PS1 CD-R?
or would that brick the console?