Food safety of custard slice
There’s a fundamental difference between food safety and what is considered acceptable risk - the former is calculated to be virtually risk-free and statistically contain not enough bacteria etc. to potentially cause illnesses or infection, plus a bit of a safety margin. The thresholds are so that even the most sensitive and vulnerable consumers are still protected, for example young children, the elderly, pregnant women and people with weakened immune systems. These consumer groups are also especially likely to suffer or even die in case of salmonella or similar food borne diseases. This is one of the reasons why pregnant women are warned against certain foods.
But there are many recipes that are not “safe” in this strict sense. That doesn’t mean that your custard, mousse au chocolat, sauce hollandaise (all with raw egg), unpasteurized cheese (raw milk), steak tartare (raw beef) etc. will automatically make you sick. In fact, in the vast majority of all cases nothing will happen. And in most other cases, a healthy adult will suffer, but survive.
To be very clear:
Just because some cooks and some recipes work outside the boundaries of food safety guidelines doesn’t invalidate these guidelines.
The recipe in the link is not safe by official standards if made with raw, unpasteurized eggs.
The temperature you found is correct without additional time constraints, there may be ways to produce a safe custard if you can hold the mixture for a certain time at a given temperature (bacteria destruction is a function of temperature and time to be exact, food safety guidelines often give a simpler version using a temperature where the holding time is near zero, so “instant” killing of bacteria), but that would be beyond the scope of the question at hand, suffice to say that this constellation is nowhere near that range.
So if you want to make the custard recipe in your link, make an informed decision, considering your personal health status and your individual risk tolerance. I personally would never serve this kind recipe for the groups listed above or for a potluck where I can’t be sure who will participate. Especially for eggs, food safety may depend on locale, and in some areas pasteurized eggs are available in stores.