With an 18-pin DIP, the first thing that came to mind was PIC1654, but the pinout doesn't match.

1 batt-
3 switch
4 switch
5 switch
6 switch
7 diode to batt- (reset?)
8 batt+
10 cap to batt-, resistor to batt+ (oscillator?)
11 LED
12 piezo
13 piezo
14 switch
15 switch
16 switch
17 switch
18 LED

Comparing two 4-bit inputs and lighting LEDs doesn't require a lot of smarts, and playing simple tunes wouldn't take much more.

Later Simon games used a custom chip, but they sold tons of those over many years. It was likely cheaper to use an MCU for a lower-volume game. Ideal Maniac used a PIC.

There might be more markings on the bottom of the chip.

I'd be happy to decap and photograph it if you want, but it will destroy it.