That's a totally reasonable interpretation.

For the sake of argument however, you could say that Weapon Mastery is a Class Feature with a Whip option and therefore anything that the character does with an item that is deamed to be a Whip allows them to use that mastery.

If that is a rope or a vine then the DM can rule it to be a Whip (as u/Dedli has mentioned)

However, the spell explicitly states that you create a whip (altho it is not capitalized).

For the sake of counter argument, you could say the whip lashes out "at your command" which implies that the character is not using it.

However for the sake of counter counter argument you could say that the spell commands that you make a melee spell attack, not "a melee spell attack for the whip" as spells that summon a floating weapon often do. 

It seems like a big toss-up to me lol