I wouldnt automatically disqualify them as many seos work entirely off word of mouth and referrals and dont need to rank well. Seos are not necessarily web designers either. That said putting together a wordpress site shouldn’t be a huge issue so I’d question why they use wix
Wix is far to restrictive for in dept SEO. The more I learn about SEO the more I realise how many people claim to be SEO experts without having any sort of in-dept knowledge of how search engines index. The best way to know of their SEO is good is to ask for a website they’ve worked on before and see how easily you can find it online without the name of the business. Also make sure they use private browsing when showing you their supposed SEO as Google will rank a website higher for an individual if they have accessed it a lot.