It's not, at least for me. If you checked in devtools, that's gzip over the wire size. Hover over the size and it'll show you the actual resource size, still 285k for me.
Sure, Discord could do a bit better, but it's not correct to knock them here for costing their users 285KB.
That's why tools like OptiPng basically brute force all the combination of options. Depending on the image content different combinations of filters and compression will get the best file size.