They extended a player in-season about 2 months ago, so clearly there is no hard and fast rule about it.
It's not the only way to see it, but one interpretation that also fits the assumptions Heyman is making is:
Stroman has publicly jockeyed for an extension with language implying he wants specifically to be in Chicago
The Cubs are the only team that can extend Stroman until the end of the season
As a pitcher, and one who technically is under contract through 2024, there's more at risk with extending Stroman earlier in the year than later in the year
We don't know how many teams will be really selling and therefore how strong offers might be for rentals at the deadline compared to recent years
So if we take all that as true, the only way it's a bad idea to wait until after the deadline to engage serious talks with Stroman is if that waiting somehow sours him on wanting to stay, which seems improbable unless that assumption about his interest in staying wasn't 100% true to begin with.
All that said though, I'm not sure I see a huge amount of upside in that waiting, because even if deadline prices are higher this year than recent years, the odds of getting a package that makes you better in 2024 than just extending Stroman seems very low.