since it was a requel, anything's possible.
i think there was one reference to 'deadpool' and repeated references to Weapon XI. so i simply say he's Weapon XI and ignore the line about 'Stryker finally figured out how to shut you up'.
and the leaked script didn't offer any explanations; it simply had a scene cut into another with a Barakapool figure with Wade's VO saying something about 'the worst day of my life' and it cuts to him talking about Wham! breaking up.