Non-native speakers can sometimes speak better English than native speakers, but on average that is generally not the case.

It's also worth pointing out that "reading" here seemingly does involve English specifically and not English and Spanish or whatever other language(s) overall.

A massive influx of immigrants who likely don't speak English natively, regardless their legality, is inevitably going to cause a drop in English literacy rates unless it's controlled for and I think it's a question worth asking because the methodology isn't immediately obvious.