The mechanic is wrong - he is mistakenly thinking of 15-inch rims. The
standard tire size for 15-inch rims on a '97 528i is 225/60-15.

For 16-inch rims, as you've already noted, the correct size is
255/55-16.

If you use 225/60-16 tires on your 528i your speedometer will no longer
be as "accurate" as it "should be". It will show a slower speed than
otherwise intended by BMW. You may more readily get a speeding ticket
and your on-board computer may go out of whack.

225/60-16 size tires may be more common but they are likely of lower
performance characteristics. Many are probably grand touring tires.

16-inch rims are actually of a commonplace rim size but BMW chose the
225/55-16 tire size for performance reasons so that is why there are
more tires available at a 60 aspect ratio vs a 55 aspect ratio.

Remember that the standard rim size for a '97 528 was 15-inch. By
definition, the tires on the 16-inch rim for the 528i is for
performance and will be more rare on the road.