By "virtuosity" he must surely have meant Hughes's well-known gift for verbal fireworks - that Hughes was to poetry what Liszt was to music when he wowed his audiences with cascades of sound. But it's possible that he was simply thinking of Hughes's craftsmanship, in a general sense.
My American Oxford Dictionary defines virtuoso as "someone who excels in the technique of doing something." It used to have an even broader meaning. In the 17th century a virtuoso was "a learned or ingenious person," perhaps a scientist, perhaps just an expert. John Evelyn was called a virtuoso in his day because he knew about fruit trees.
Then somehow or other its meaning got narrowed down. A virtuoso became a connoisseur of the arts. Virtuosi of this sort did not, on the whole, get a good press. The OED says "frequently one who carries on such pursuits in a dilettante or trifling manner" and quotes an octogenarian Thomas Hobbes as saying crustily that "there be many men called critiques, and wits, and vertuosi [sic], that are accustomed to censure the poets, and most of them of divers judgements".
Already we begin to detect, too, that peculiarly English distrust of aesthetes that was to achieve its full flowering in the late Victorian pages of Punch, with its popular cartoons by du Maurier showing long-haired, chinless, flat-chested young men contemplating a rose or holding a lily; one of them is seen asking the waiter for a glass of water. "Would there be anything else sir?" asks the waiter. "That will be all thank you," says the exquisite youth, putting a rose in the glass. You couldn't find anyone less like our Ted, whose physique was rugged and whose jaw was firm.
But Ted was a different sort of animal anyway. The old virtuosi weren't usually practitioners as he was. They were mostly the onlookers, what Shaw later called "those who can't". The idea of virtuosi as performers doesn't seem to have taken proper hold till the 19th century.
Even then they were often thought to be, well, not quite British. Virtuosity - Mr Seitz's word - was lowly regarded; it was not a virtue. ("Excessive attention to technique," says the OED.) Virtue came from the Latin virtus, which meant valour or manliness, having itself come from vir, the Latin for a man. But those who spoke of virtuosity had forgotten its masculine pedigree; if anything, they thought of it as effeminate.
Why virtue should have started by being something that men, rather than women, were expected to show is a shameful question that has already been fully answered by feminists. A virtuous man was brave, but even as late as the middle of the last century a virtuous woman was required only to be chaste. If we don't hear virtuous used much now, perhaps it's only because it has the wrong echoes.