Even though many arpeggiators sound monophonic, in the vintage days, true arpeggiation was, as far as I know, always achieved with a scanned polyphonic keyboard, such as the Jupiter 4, which I understand was the first to feature it. What comes out is sometimes monophonic but the input is polyphonic.
You would play a chord and the circuitry sends out a pattern from all the notes that are on.

In modern times the best way to do that if you don't have the dedicated hardware is to send a MIDI output into some software that turns it back into MIDI or more usefully to your modular, either directly into CV or converted into CV.

That said, a lot of vintage recordings I used to think featured arpeggiators are actually sequencers being transposed with keyboard voltage. A good example is Vangelis' "Spiral"
From a performance standpoint it's not the same thing at all. You can arpeggiate a chord or chords and then play it live in different keys with a single key press. Sequencers like the Doepfer let you reverse and randomize, but you can't turn a major chord into say a diminished 7th chord without stopping and programming in that new chord (or using some elaborate quantitizing that still won't add extra notes to make a triad chord into say an 11th chord )

Nick