According to the article, there were actually conflicting guidelines.
The FAA said always follow TCAS ("any guidance from TCAS overrides guidance from ATC"), whereas other regulators were either vaguely non-committal to which had overriding priority or assigned priority to ATC.
It also notes that the Russian pilots were not accustomed to TCAS (which was not common in Russia, at the time). Even further, to the extent they were familiar with it, Russian regulations noted that the ATC's guidance superseded all other guidance systems.
What the FAA had to say on the matter isn't relevant, and doesn't contribute to there being "conflicting guidelines". The FAA doesn't regulate European airspace.
Sorry, I didn't mean to suggest that the pilots were operating under conflicting regulatory guidances.
Rather, I was pointing out that the international community of aviation regulators had established conflicting norms with regards to the requirement and prioritization of TCAS at the time of the incident.
The conflict in norms was a contributing factor to the incident discussed in the article, and in the aftermath of the incident, those norms were appropriately corrected. The mention of the FAA is relevant because their original guidance (TCAS guidance supersedes ATC guidance) was what the ICAO adopted in response to this incident.
The guidance and sole purpose of TCAS is "do what it says", in that respect it is no different from a stall warning.
The fact that Russian aviation failed to train (or even require training for that apparently) is absurd, the fact that the EU allowed planes to travel with TCAS but not with pilots that had the most basic understanding of TCAS is a sad example of the results of checkmark based safety.
As even this article acknowledges, TCAS as a system makes no sense of any kind if it can be countermanded by someone other than the pilot. There is no use case for TCAS that makes anything other than "do what it says, a fast as possible" have any value at all. Disobeying TCAS because of ATC is no different than ignoring a stall warning because ATC told you to climb.
When it was introduced - and it was new at the time - not everything was as neat and clear as you state.
Quoting Wikipedia: "The [TCAS] manual described TCAS as "a backup to the ATC system", which could be wrongly interpreted to mean that ATC instructions have higher priority."
In fact, you'd do well to read the entire Wikipedia article to see that this was not the only case where pilots did not have clear guidance on which orders to follow in the case of conflicting ATC and TCAS orders.
The FAA said always follow TCAS ("any guidance from TCAS overrides guidance from ATC"), whereas other regulators were either vaguely non-committal to which had overriding priority or assigned priority to ATC.
It also notes that the Russian pilots were not accustomed to TCAS (which was not common in Russia, at the time). Even further, to the extent they were familiar with it, Russian regulations noted that the ATC's guidance superseded all other guidance systems.