This is one of the things you quoted:

can only run silently in the electric-only mode, submerged. And because that’s off batteries, your submerged endurance is limited.

Also in the article:

So, unique to nukes, are a bunch of pumps, turbines, and reduction gears, which a diesel boat in battery mode doesn’t have at all.

Ie. a diesel sub is quiet in electric mode, potentially quieter than nuclear subs because in battery mode there aren't turbines or pumps whining away, even though that is for a limited time.

Another example:

Stirling engine is particularly well suited for a submarine because the engine is nearly silent and can use the surrounding sea water as a heat sink to increase efficiency. Submerged endurance is dependent on the amount of liquid oxygen stored on-board and is described as "weeks". The class is characterized by its low acoustic signatures, extreme shock resistance, and a competent combat system. .... The class has many features that enhance stealth, helping it to remain undetected. All shipboard machinery is isolated and mounted on rubber dampeners to reduce vibrations and noises; a hydrodynamic hull design reduces noise, infrared signature, and active sonar response. Its magnetic signature is counteracted by 27 independent electromagnets, short circuiting extremely low frequency (ELF) electrical fields. Various hull coatings reduce active sonar response, and the mast is coated with radar-absorbent material. Combined with the near-silent operation of the Stirling generator and slow-turning propeller to prevent cavitation, the boats are very difficult to detect under water, especially in their normal area of operations, the Baltic Sea

https://en.wikipedia.org/wiki/Gotland-class_submarine

The article goes on to say:

In 2005, HSwMS Gotland managed to snap several pictures of USS Ronald Reagan during a wargaming exercise in the Pacific Ocean, demonstrating that it was in a position to sink the aircraft carrier.[11] The exercise was conducted to evaluate the effectiveness of the US fleet against diesel-electric submarines, which some have noted as severely lacking.

TLDR: an advanced/modern diesel submarine is louder than a nuclear submarine in normal operation. Chugachugachugachuga. Then it goes into battery mode. It goes near silent, quieter than many nuclear submarines which still have turbines, pumps, etc. The diesel sub dissapears like a ghost. Torpedo away, too late.

And that's a relatively old sub. The French sub I mentioned is something like a decade newer, with newer and better batteries.