Thats just total nonsense, the 10 refers to the cold temperature equivalent thickness, if its mineral or synthetic that is still true, the 40 refers to the hot temperature equivalent thickness, again for both mineral and synthetic.

So a 10W40 mineral would be thicker hot than a 5w30 synthetic if you were going to make the assumption that the 10w40 he has used was mineral like you seem to want to and that the 5w30 was a synthetic.
So it wont be "hot too thin" at all, in fact if will be thicker.

In reality though its very rare to see anyone selling straight mineral oil at all these days in 10w40, thats a grade you tend to find is normally semisynth if anything, but it could potentially be anything upto and including the latest ester synthetic.

In one of our trackdays cars for example I run 15W50 Motul 300V for example, thats a good quality fully synthetic oil ( Double Ester in fact) with a great additive pack and fantastic tolerance to high temperatures without breaking down, and yet you would dismiss its quality on ground of the thickness?

I really think you have completely misunderstood what the numbers actually mean, they tell you nothing at all about if its a mineral or a synthetic.