The transmission lines run to solar farms should be able to take 100% of the output of the farm and then some otherwise the farm was over built and wasted money.
At short distances from the solar farm, yes. But it's not 100% in every direction for arbitrarily long distance. At some point, you assume the energy will be tend to be used sort of near where it's generated.
To put it another way, if you build solar between city A and city B, would you build it so it can still be fully utilized even if city A stops using any power and city B wants all of it? No, you assume city A is always going to need some power.
Generally they connect directly to large back bone transmission lines that carry power far beyond the local area though not directly to more balkanized power zones. On a large scale yes if critical junctures go out the rest can't take the full load but that's different than a single plant being overspecced for the transmission capability it's connected to.
That's not necessarily optimal. For home installs, you can overbuild panels because they're cheap compared to the inverter. Then you curtail sometimes at midday and have extra energy on cloudy days and in the morning/afternoon. Turns out that's more cost effective than sizing the panels perfectly. The same logic could apply to utility farms, because transmission lines can be expensive. I don't really know myself since I don't work in the industry, but I would not be surprised if they slightly overbuild vs. the transmission line capacity.
If by output you mean the maximum output of the inverters. The maximum output of the PV modules can be higher, particularly if the field has integrated batteries.
By this definition all reserve capacity is wasted money. That is clearly false, as demonstrated by the event in this article _not_ leading to failure and harm.
I'm not talking about reserve capacity at the grid level. I'm talking about excess capacity at the individual generation plant level that exceeds the grids capacity to take in. If you can't output the energy onto the grid the only benefit is for local maintenance and you don't need huge amounts of excess capacity to solve that and that excess doesn't help in the event of a large base producer like a nuclear plant going offline because it can't get onto the grid!
