Dyrrn’s Tentacle Whip is a whip, which is a melee weapon.
The Whip appears in the Martial Melee Weapons table of the PHB. Dyrnn’s tentacle whip says:
Weapon (whip), very rare (requires attunement)
So Dyrnn’s Tentacle Whip is a melee weapon. When attacking with a melee weapon, you make a melee weapon attack, not a melee spell attack.
The Tentacle Whip does not extend your reach for melee spell attacks.
The Reach property of weapons says:
This weapon adds 5 feet to your reach when you attack with it
If you are casting a spell that calls for a melee spell attack, you are not attacking with Dyrnn’s Tentacle Whip so the Reach property of the weapon does not apply.
Of course, a DM could rule that it does count, and from a flavor perspective this seems reasonable, but that would make this magic item extremely powerful, far more powerful than it is otherwise.