An independent diesel generator could have been loaded onto the ship as easily as the spy equipment. It was a tanker, there was plenty of room for a genset and its own fuel. Then the ship's power plant wouldn't be taxed at all.
Then you have radio equipment in addition to an independent diesel generator and fuel to explain…
Yes it was a tanker. The bigger the ship doesn’t mean it has more people or more power generation like a cruise ship. A tanker has a crew of 4 plus captain, maybe?
You've got the spying equipment to explain if boarded. An extra generator is hardly a concern. If anything an auxiliary generator can be explained as a backup/replacement for the ship's mandated backup generator.
Generally the people ordering radios don't think about that. They are used to land where you have more power to the building than you could possibly use.
