I do not know much about the architecture of Rational/R1000s400, but despite that I am pretty certain the claims that it was particularly good for implementing Ada on it were not true.
Ada can be implemented on any processor with no particular difficulties. There are perceived difficulties, but those are not difficulties specific to Ada.
Ada is a language that demands correct behavior from the processor, e.g. the detection of various error conditions. The same demands should be made for any program written in any language, but the users of other computing environments have been brainwashed by vendors that they must not demand correct behavior from their computers, so that the vendors could increase their profits by not adding the circuits needed to enforce correctness.
Thus Ada may be slower than it should be on processors that do not provide appropriate means for error detection, like RISC-V.
However that does not have anything to do with the language. The same problems will affect C, if you demand that the so-called undefined behavior must be implemented as generating exceptions for signaling when errors happen. If you implement Ada in YOLO mode, like C is normally implemented, Ada will be as fast as C on any processor. If you compile C enabling the sanitizer options, it will have the same speed as normal Ada, on the same CPU.
In the case of Rational/R1000s400, besides the fact that in must have had features that would be equally useful for implementing any programming language, it is said that it also had an Ada-specific instruction, for implementing task rendez-vous.
This must have been indeed helpful for Ada implementers, but it really is not a big deal.
The text says: "the notoriously difficult to implement Ada Rendez-Vous mechanism executes in a single instruction", I do not agree with "notoriously difficult".
It is true that on a CPU without appropriate atomic instructions and memory barriers, any kind of inter-thread communication becomes exceedingly difficult to implement. But with the right instructions, implementing the Ada rendez-vous mechanism is simple. Already an Intel 8088 would not have any difficulties in implementing this, while with 80486 and later CPUs maximum efficiency can be reached in such implementations.
While in Ada the so-called rendez-vous is the primitive used for inter-thread communication, it is a rather high-level mechanism, so it can be implemented with a lower-level primitive, which is the sending of a one-way message from one thread to another. One rendez-vous between two threads is equivalent with two one-way messages sent from one thread to another (i.e. from the 1st to the 2nd, then in the reverse direction). So implementing correctly the simpler mechanism of sending a one-way inter-thread message allows the trivial implementation of rendez-vous.
The rendez-vous mechanism has been put in the language specification, despite the fact that its place would have better been in a standard library, because this was mandated by the STEELMAN requirements published in 1978-06, one year before the closing of the DoD language contest.
So this feature was one of the last added to the language, because the Department of Defense requested it only in the last revision of the requirements.
An equivalent mechanism was described by Hoare in the famous CSP paper. However CSP was published a couple of months after the STEELMAN requirements.
I wonder whether the STEELMAN authors have arrived at this concept independently, or they have read a preprint of the Hoare paper.
It is also possible that both STEELMAN and Hoare have been independently inspired by the Interprocess Calls of Multics (1967), which were equivalent with the rendez-vous of Ada. However the very close coincidence in time of the CSP publication with the STEELMAN revision of the requirements makes plausible that a preprint of the Hoare paper could have prompted this revision.