So they didn’t exploit flaws, they just tried to work around the protection.
Even though the Gameboy is relatively slow (~4.7 Mhz processor IIRC) it’s still not going to be easy to hit exactly at the right time to be able to perform your fault injection.
The protection is simple enough to be effective. If you did it with a modern manufacturing process it’d probably be quite difficult to be able to image the bits directly...
Nowadays, for say Nintendo Switch, you are more likely to see keys and firmware data dumped via early boot exploits or privilege escalation.
Fault injection is a method to cause microprocessors to execute instructions "incorrectly" by manipulating external factors (clock frequency, input power, etc). Getting the CPU to fail to write to a memory address, or flipping the truth of a comparison instruction, and other "glitches" can influence execution of security-sensitive code and ultimately undermine security mechanisms.
The original Gameboy ROM was dumped by physically de-capping the chip and reading out the bits: https://dot-matrix-game.blogspot.com/2014/01/boot-roms.html
tl;dr For the SGB, they quadrupled the clock speed briefly to glitch over the one instruction responsible for mapping out the ROM.
For the CGB, they halted the clock, drained the voltage to randomize the instruction counter, then used a NOP slide that led to their payload.
This simple security mechanism was extremely effective, and lead to the DMG BIOS being undumped until relatively recently, when it was dumped by imaging the ROM directly and via fault injection to skip the last instruction.