I mean, probably? There's definitely a statistical likelihood of folks who use cannabis also using tobacco products or other substances. It's not a "gateway drug" so much as a "anyone who enjoys thing X is likely to try related things" object. I recall those early studies about alcohol in moderation being good for you eventually being debunked as, "actually, folks who could afford alcohol consistently in moderation also ended up being able to afford health insurance and that created the better outcome"; I suspect there's something similar with cannabis when we look retrospectively, given its illegality for so long (users are less likely to seek medical attention for symptoms or precursors to avoid the stigma).

The thing is we don't know without doing more research, which I genuinely appreciate the authors essentially calling out point blank. They're not saying cannabis is bad, they're saying that looking at the thin amount of valid data we have available thus far, there's definitely a correlation there worth investigating further.