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I've seen it written that a dent in the top tube depending on it's size can be dodgy strength wise.

Drilling holes in top tubes to run cables through how does that compare again strength wise or are the holes reinforced?

Dent in t/t will hardly have influence on the strength.
Only if it's a big dent (fist will fit into it) with sharp edges (almost possible to see cracks), it's reducing the strength.

Holes: if these are big enough (e.g. 5mm or bigger) and the wall thickness is small (steel 531, SL, etc), a drilled hole can lead to breakage. Will not take that long.
However, if it's reinforced, e.g. with a bottle boss or the like for a race number, it's no problem at all.
Can even be a reinforcement.
There are computer models for that.
I wouldn't be afraid of a correctly drilled/reamed hole with a dedicated cap as in the Toei picture. That's pretty common. Not more dangerous than any other braze-on part on any tube. Important is how it is done. E.g. not with brass or copper...