I don't know anything about tanks but reading the article about the Slovenian armour the author claims

... the M-55S isn’t really a T-55. It’s the bones of a T-55 with a new brain, new muscles and new skin.

And he gives a list of all claimed "upgrades". So, with a charitable interpretation the double-standard goes away.

Then again, even if the M-55S isn't as antiquated as an unmodded T-55 its not like war has age-restricted combat where all the elderly tanks get together and fight it out in a safe zone.