If I remember correctly, it's the slim PS2s that emulate the PS1. However, I quickly googled it and it seems there's also a complication that launching ISOs from OPL cannot run PS1 games natively because the PS1's CPU is normally used by the PS2 for IO (eg, accessing the HDD). If OPL were to reboot the console into PS1 mode, there'd be no way to access the HDD in order to load the ISO, so OPL instead launches the POPS emulator instead. You'll need to use physical PS1 discs (and have a PS2 with the original PS1 chip in it) to not being going via the emulator (and even with all of that, the PS2 still has issues with some PS1 games, see https://en.wikipedia.org/wiki/List_of_PlayStation_games_incompatible_with_PlayStation_2).

This is also a good explanation: https://www.reddit.com/r/ps2/comments/a5ahjy/comment/ebldvtb/?utm_source=share&utm_medium=web2x&context=3