The Torah says you can't cook milk and meat together. Thus from the torah the prohabition is only if you cooked them together (but rabinically it is prohibitted in all mixtures, even cold)
Ther is a amoung the Poskim wether roasting is included in cooking and is prohibited from the torah or it is only rabinicly prohibited.
A proof they bring that roasting is include in cooking is from Perek 16:7 That says on a Korbon Pasach (the Passover sacrafice), which has to be eaten roasted, "You shall cook it" .Since the torah used the term of cooking to explain roasting , so roasting is included in th term " cooking"
In my humble opinion, the poskim that hold it's only rabinical would say the following:
There is 2 types of way a word is used. The technical way, what it realy means. Then ther eis a looser meaning. If someone is giving directions for something, for ex. a recipe, you must give the technical term (If you say that something that you need to roast you write "cook it" it won't come out very well. )
If you are writing a story format, then you can use the looser term, like "he cooked supper", even though technicaly he roasted supper
So, here in R'eah, which is not the main place that talks about the laws of korbon pesach, but rather it's brought here to be mention with the other laws of the holiday Passover, so it can use the loose term of "cooking". but by the laws of milk and meat, where these are the laws , therefore it uses only technical terms and is only includes cooking and not roasting