Is it polite?

“For safety reasons, we couldn't borrow an extension cable, therefore a colleague  of mine recommneded the nearest store. It was truly a gesture of generosity,

We really do value about you as our guest and we genuinely want you to know that we didn't intend to be discourteous.”

For reasons of safety, we can't borrow an extension cable, therefore my collegue recommneded the closest store. It was a gesture of kindness,
We really do care about you as a guest and we truly want you to know we didn't mean to be rude.

It is dependent on your point of view, gesture of kindness I would not agree, reasons of safety I doubt it. Being rude I don't think it is impolite to refuse any offer. Whether it is asking for a lolly or to borrow a car. Saying no to any offer is a right, a choice and is not being impolite. How you say no is where being impolite becomes possible. In the words we use or the tone of your voice a simple no can become an insult.
Words in themselves are just words but how we present, deliver them is more significant than the words themselves. With my tone I can be respectful or condescending or just down right hostile.

Is this polite?

For reasons of safety, we couldn't borrow (an) extension cable, therefore my colleague (the proprietor/manager/receptionist) recommended the closest a store close by. This was a gesture of kindness.

He/She said, "We really do care about you as a guest and we truly want you to know we didn't mean to be rude."

- It sounds like a polite excuse to me.