I've certainly come across people using 'WiFi' to mean internet generally, and given the branding of 802.11ax as "WiFi 6"[1] it's understandable that people might consider it the successor to "5G". I recall someone recently had a phone salesoik insit that 4G was the same thing as WiFi (which is why 5G is better) but that's the sort of thing you expect from salesoiks[2]. I expect the teenagers will continue to understand the difference, if only on a who's-paying-for/censoring/throttling-what level. Meanwhile there must be plenty of people like my MIL, whose mental model doesn't even extend to the difference between an SMS and a message sent over an internet connection, and are likely to use terms at random.
[1] 802.11ac gets retconned to 'WiFi 5' and 802.11n becomes 'WiFi 4' and so on...
[2] I had one tell me that computers were getting out of date in 1991, apparently because Super Nintendos[3] were faster and better.
[3] A childhood in which Super Nintendos were new and exciting now qualifies you as middle-aged. Terrifying thought.