TCAS has no speed input at all. Honeywell uses the attitude input to select which TCAS antenna to use for target tracking. Collins uses relative signal strength instead. TCAS is interested in rate of closure.
So it would depend on the TCAs model - I don't remember if someone wrote about it in this thread - in this particular a/c the need of a direct attitude input ?
I wonder how TCAs gets
altitude information... GPS only, GPS with IRS, IRS only, baro/altitude input from ADRs (with or without eventually computing airspeed (pitots) for adjustements ?)