I was also outside of Canada when I received my PR approval email, and was planning to return only in 4 weeks. I ended up replying to the email, mentioning my expected return date, as well as attaching my return flight as a proof. IRCC responded on the same day and asked me to notify them once I'm back. After I landed in Canada 3 weeks later, I sent IRCC a follow-up email, and they sent me the portal credentials within a few hours.
tldr: it should be safe to respond