Base plate, too little cement in the concrete

Hello, I am quite clueless when it comes to construction and am very worried about the following problem:

I want to build a 3x5x1.5 swimming pool in my garden using Styrofoam formwork blocks that will be poured with concrete.
Over the weekend, a friend who is a construction expert helped me pour the base slab (20cm), and for various reasons, we had to mix the concrete ourselves from cement and gravel.

I read online that the ideal mixing ratio is 1 part by weight cement to 4 parts by weight gravel, and that is exactly what the friend told me beforehand.
He mixed the concrete and I poured it.

Now that the slab is finished, I checked the delivery notes.
If I recalculate everything, we used exactly 8 tons of gravel and 1.250 tons of cement.
At a ratio of 1:4, it should have been 2 tons of cement.

He says that is no problem and comes up with arguments that I don’t fully understand.
(for example, there was still water in the gravel, so we actually used less gravel than was delivered.
So it was indeed moist, but as a layman, I am still very worried whether it will really hold.

Does anyone know about this and can please tell me something?

No one will confirm that to you

There is not just one concrete and one cement, but hundreds of different mixtures.

No one is to sign for me that it holds. I would like the non-binding opinion of someone who knows their stuff. What would you do in my place?

Have the type of cement given to you and ask the manufacturer.

Is there at least iron included and was it compacted? If proper iron was installed and the concrete was well compacted, I would not worry.