So first and foremost, how do you know that it wasn't deliberate? It's not like there wasn't a reference to how to write the title in their database.
Secondly, why must it be in a GudaGuda event? They could just make them 1/2* Servants and toss them into the FP pool. Solves both your desire to want them and DW's resolve to stop making more GudaGuda past GudaGuda Final. It's not like we didn't have a precedence of having Angra Mainyu or the recently added Saber Lily into the FP pool, both weren't from the main story and yet still rollable from FP pool. Yes, we could argue that Saber Lily being added to the pool was because she was a pre-reg Servant and all that but it still doesn't fully discount it.
Lastly, do they *have* to be added? If that's the case, we should've had Riyo's other two Servants that she initially made herself. The Assassin one and the other one whose class escapes me. Or heck, remember Sita. 4 years now. Still no Sita. What about the tease on Vishnu? We had a personality dump with Rama's interlude but nothing came from it after that.