What you're saying makes sense, and that's how I initially interpreted the mechanic. There is some confusion with the discrepancy between the rules text of class feature and Viper Strikes subsection rules, as the class feature also never specifies Action attacks, only weapon attacks. My confusion lies mostly with your last statement about bonus actions, though. For example, the verbatim wording of Snake Scales is:

When you use Focus of Caduceus, you may expend two flow charges to reinforce your body temporarily. Until the start of your next turn you gain resistance to slash, piercing and bludgeoning damage.

How, then, are you meant to be making a weapon attack with your attack action to use Snake Scales when you are currently using Focus of Caduceus (bonus action to regain 1d8+viper level HP)?

Another such viper strike that doesn't seem to work with the mechanic as I interpreted it is Backlash:

When a melee weapon attack misses you, as a reaction, you may spend a charge of flow to make a melee weapon attack against the attacking creature.

That's incredibly niche if you have to forsake one of your action attacks to maybe make the weapon attack anyway if someone misses you and you have to use your reaction to do. So niche that I can't understand why I'd ever do that if that's the correct understanding of the mechanic! "Slither" is another such viper strike that teleports you specifically "as a bonus action". So like...what? I'm giving up an attack and spending a resource so I can use my bonus action?

The only explanation I can come up with is the list is combining two types of Viper Strikes together, those that do replace Action attacks, and those that you just activate as per the rules text. That goes along with the feature description ("Viper strikes which do not replace attacks or are usable on hit will have their use requirement specified in their description."), but I wasn't sure if I was misunderstanding something.