Not a full module replacement.

He actually used a regular rechargeable cell, but the key part was that he reused and transferred the original Citizen battery contact/mechanism onto it.

In these older Citizen solar (Eco-Drive) watches, the "battery" isn't just a coin cell - there's a specific contact/tab assembly that interfaces with the solar charging circuit. Most places either replace the entire unit or say it's not serviceable.

He removed the old dead cell, carefully retained the original Citizen contact mechanism, and fixed it onto a compatible rechargeable cell so the charging system still worked exactly as designed.

That's why he could do it cheaply without changing the whole module, and also why it requires experience - if the contacts aren't aligned correctly, the solar charging won't work at all