Can I use MacBook Pro's 70W USB-C power adapter to charge iPhone 17?

Yes, you can safely use your MacBook Pro's 70W USB-C power adapter to charge your iPhone 17. The iPhone will only draw the amount of power it actually needs, and USB-C Power Delivery technology handles the power negotiation safely without damaging the battery. 

See this link for further reference

Fast charge your iPhone - Apple Support

Watt  is like how fast your car can drive, but the speed limit is how fast your car do drive

the iphone draw as much current as it was build to use when charging, the fact that

a charger can provide a lot more just means it's not having its full capacitive used

so as long as it's no higher than 5volts and the connector fits and it's minimum 5 watt then it could be 5GigaWatt without problems

I guess that same staff member might opine that you can’t plug a 50-watt desk lamp into 2,400-watt wall receptacle. 🙄

Training staff is always a challenge.