I use a similar rationale when I make offers, although, "comps" can be a little subjective, depending on which ones you decide to use.
It's not uncommon for people to point out the assessed value, thinking it's an appraisal (which it definitely isn't). It usually just takes a bit of education, and most people are willing to listen.
I'll usually explain that the assessed value is basically a wild guess from the county or city about what the property is worth, but the county is not an appraiser. They don't understand how to objectively value properties and if anything, they have a huge bias toward inflating this number as high as possible, because the higher this number goes, the more they can charge in taxes.
If a person can see that the assessed value is not an indication of a property's fair market value, this number can effectively go right out the window.
Of course... this still doesn't change the fact that your offer is still going to be very low (there's no way to sugarcoat this), but psychologically, it does help to get rid of this number as a valid anchor for the property's value.