should lemon be cooked in stainless steel?

Lemon curd will not react in a stainless steel container, so it will not impart a metallic taste. You can use stainless steel, enamel coated pans, or heat resistant glass. This extends to the other implements you would use as well- the spoon, spatula, and a sieve if you used one. I imagine the reason for suggesting the stainless steel bowl was because heat resistant glass bowls are not found in every kitchen. Plus there is some trouble using modern glass bowls because of the type glass they use these days- which leads them to shatter unpredictably and impressively at inconvenient times. That isn't a problem with stainless steel. You need to know the type of glass your bowl is made of, borosilicate or lime soda- because it can make a big difference.