Blizzard should rotate or delete this card already, it's bad for everyone. Who plays and rely on it can be screwed and who play against doesn't like it too, cause it can remove ur board very easy and stupid. This may solve Zephrys problems!
I also had a situation where my opponent had 9 HP and I had above 9 dmg lethal, but opponent had 1 taunt minion. So I figured it would give me Silence or Mass Dispel or any other silence card to silence 1 minion and kill my opponent but it didn't. Can't remember options but no lethal from it. Ended up putting himt to 1 HP, but still winning the game at the end. Just wasn't playing Highlander Hunter but Priest instead.
It might evaluate "minions summoned this turn (can't attack)" the same as "minions who already attacked this turn (can't attack)." Otherwise, you might run into situations where your only minion (with Windfury already) attacks twice, and then Zephrys offers you Windfury because it thinks that's lethal.
Certainly, things COULD be coded to count "# of attacks this turn," but I doubt it already is. And changing it just to suit Zephrys' logic isn't a great use of dev time. So the general guidance re: Zephrys remains the same -- if you're angling for a specific card/solution, set things up as best you can to encourage that outcome. From this, I think we've learned that Zephrys probably evaluates the number of minions who are currently eligible to hit face when deciding whether or not Windfury is a vector for lethal. So apply your buffs and clear any taunts first (if possible), then let Zephrys tally up as many eligible attackers as possible.
You could've also kill commanded first and play zeph then for a bloodlust, which would also be lethal. Windfury would actually be the better card here, so I'm not saying you missplayed or anything. But bloodlust is just easier for zeph to spot.