Anyone who went to drama school, or to drama classes, as Kelvin Fletcher did, could possibly be described as a 'trained and experienced dancer' (simply because dance normally forms part of the curriculum at such institutions). Are you therefore suggesting that all trained actors should be barred from competing in Strictly?
Further, Kelvin Fletcher was brought in as a last-minute replacement for Jamie Laing. It can't have been easy for Strictly's production team to find someone with sufficient dancing skills to compete (without much of the initial training that the other competitors had received by then) and who was sufficiently well-known to be recognised by the show's audience (but with several weeks free in his schedule to take part in the programme). Their choice will have been extremely limited!