Verdict: Misleading
A Wi-Fi extender can improve usability in a weak-signal area by extending coverage. That does not mean it increases the speed supplied by your ISP.
Why placement matters
If you put an extender where the existing Wi-Fi is already extremely weak, it has a poor signal to repeat. Better placement can matter more than buying a more expensive unit.
When another design is better
If Ethernet is available, a wired access point may provide a stronger result. For several coverage zones where cabling is difficult, a well-designed mesh system may be more appropriate.