Yeah it's a general rule, there are some exceptions, but they're less common than is worth challenging this claim for. Some drinks would be too strong to be made from a double, these will use a split single in countries where doubles are the norm. The second shot will end up wasted unless someone else is ordering the same exact drink or ordering a single espresso (also rare outside of Italy and some other countries). I've seen it but most customers prefer to see bigger drinks these days so most cafes don't usually face this problem and make drinks from double shots.
For reference, I use a 36g shot (basically 36ml if you wait for the crema to dissipate) for a 170ml "flat white" at home. This achieves a balanced strength for this kind of drink. You really have to get into the tiny drink category before you really reach the realm of "this is too strong and I must use a split single". And, in those cases, due to the fact of the double shot (and, I would struggle to come up with "sources" but it's again generally accepted that unless you can use the second split shot for a drink immediately, it goes down the sink), you are still often using a double worth of coffee for a single shot drink.
For reference, I use a 36g shot (basically 36ml if you wait for the crema to dissipate) for a 170ml "flat white" at home. This achieves a balanced strength for this kind of drink. You really have to get into the tiny drink category before you really reach the realm of "this is too strong and I must use a split single". And, in those cases, due to the fact of the double shot (and, I would struggle to come up with "sources" but it's again generally accepted that unless you can use the second split shot for a drink immediately, it goes down the sink), you are still often using a double worth of coffee for a single shot drink.