Yeah, that is almost certainly a bit high (unless the regular load of riders/pillion/gear on your Spyder is up over about 550 lbs?!

) so you may well have worn the middle of your rear tire more than is ideal, possibly in a more heavily worn 2-3 inch wide strip in the centre of the tread - but that will depend somewhat upon things like the ambient temps when you ride, how long you ride for, and the road surfaces you've been riding on...

Are the edges of the tread still good, while the middle of the tread is worn down to/beyond the tread wear indicators?
Any wear reaching the tread wear indicators
anywhere across the tread face, even if it's just in one little spot, means the tire's worn out and should be replaced!
For all but the heaviest loads/riders &/or hot & long distance riders, I'd suggest that about 18 psi is as high a pressure as you'd want to run in just about any a/mkt rear tire that'll fit our Spyders in OEM trim,
IF you don't want to shorten your tire's potential life due to excessive wear in that middle strip. Sure, the 205 wide tires will be a bit less prone to this than a 215 or 225 wide rear tire, but like I said, 20 psi
is really a little bit high...
But you say you're not even close to down near the tread wear indicators yet tho, so why does the dealer say you need a new tire??

Or is it only on the edges of your tire's tread that you aren't worn down?? Or are there some other wear or change indicators coming into play here??
A tire that was new in 2021 shouldn't be 'old' enough to warrant changing except for milage related wear; but uneven or patchy wear, scalloped tread blocks, heel/toe wear or feathering on the outer edges of the tread (inner, outer, or both!), or even just one side of the tread being worn more than the other could all point to a need for a new tire, beyond that worn down centre strip, of course!!

However, for anything but the previously mentioned centre strip wear that'll likely be due to over-inflation,
I'd want the underlying cause of the abnormal wear to be rectified by the dealer
FIRST,
before replacing the tire, or you'll risk stuffing up the new tire too!!