Look around from a rooftop, do you see many factories with smoke coming out of chimneys?

Yes: https://maps.app.goo.gl/1ZDRiQdhr3Ngf5qP6

I'd not be surprised if this activity is quite limited in Singapore given that they have been under a tight microscope lately. Money laundering used to be their business but now they are rich and make money "legitimately" and within the "international" framework.

To be fair, having an industrial district doesn't preclude tariff laundering either. If you're trying to pass chinese goods as singaporean, you'd want your address to be a factory, rather than an office in a business park somewhere. Even if that wasn't a consideration, at the very least you'd need warehouse space to repackage/reship goods, so you'd need to rent out a warehouse/factory anyways.

Most factoies don't have a chimney. electric powers them.

You'd be surprised. Look up Jurong Island.