You are correct, that shop is completely wrong.
Even the free avast version offers more protection than MS-Defender.
avast wil have no problem with the coming update for Windows 10.
Look it up...
Does MS-Defender has.. ?
- FileShield
- Webshield
- Mailshield
- CyberCapture
- SafeZone browser with pay-mode
etc...
I think you already know the answer
When I had my computer service shop, I've seen many of those shops.
None of them existed long.
A huge clue about that shop is that they have enough time to offer free cleanup's.
It means they don't have enough customers to fill their time.
Not surprisingly ofcourse if they say Defender is better than avast
If you want to have a laugh, go back to them and ask them what CyberCapture or a SafeZone Browser is.
It would not surprise me they don't even know.