It was them in life
In general (with the exception of like, literal gods and those who scale to them) a servant is the same strength as they were in life without outside nerfs or whatever, this is mentioned in CM3 and we use this generally and will use stuff mentioned from their backstories while alive
KoTR servants unless being nerfed like when Shirou is a master, are similarly as strong as they were in life, we get that statement with Saber when she finally gets Rin as a master,, and then the other KoTR servants maintained their relative strengths in comparison to her and eachother
The closest to them fighting as servants though was Lancelot, Bedivere and Gawain fighting Tristan who had Inversion, Rampage, and Nightless (which includes Numeral of the Saint based on what Higashide and Nasu have said in Bamboo Broom) and using their NPs during the fight