Your first 70 achievement points will most likely be your first 70 levels. Every other achievement requires you to be in a Level 70 game. Not that you can’t get 70 achievement points before Level 70, but that would require a Level 70 carrying you. And at the beginning of the season (i.e. first 90 minutes), that’s most likely not happening. It also records a time stamp.
As an example, for Season 27:
(All times in Eastern):
On Asia, TongSu was the first player to hit Level 70 at 4:51:12 AM
On Europe, Lybrix was the first player to hit Level 70 at 11:44:48 AM
On Americas, VelaRUS was the first player to hit Level 70 at 8:51:47 PM
Lybrix is the only play to hit Level 70 in under 50 minutes more than once, in which he did it twice, once in Season 26, once in Season 27. As mentioned above, at the 75 minute mark, only 85 players had hit Level 70.
On Americas that season, only 9 players had reached Level 70 at the hour mark:
https://i.imgur.com/GoErpRg.png
Starting at the 30 minute mark, I check every 5 minutes up until the 75 minute mark, at which point I check every 15 minutes up until the Achievement Leaderboard has 1000 people at 70.
Dabooty (Player #10 in the above) hit Level 70 at 9:00:16 PM. Close, but he still didn’t make the hour cutoff.
Now, it is possible that the leaderboard could not record everything perfectly, but with everyone who streams, the time matches, with it only being 1-2 seconds off at worst.
EDIT
As far as dual classing your way to 70:
I recorded this today, doing the entire Season 29 Journey. Started Necro and switched to Wizard, completing Haedrig’s Gift at roughly 3h2m. No deaths, and everything kept within season mat confines. I’d probably add 15 minutes though to simulate finding a Vision, as I did take that approach. Still, 3h20m to a full Haedrig’s Gift as a wizard is fairly competitive.
(Technically, I over gambled at the start, but I would have gotten the Grasps of Essence within the 475. The very first run though, I restarted at around 2 mins in, as I actually whiffed. You don’t really have a large chance whiffing though, so I’m looking at a more realistic take, rather than a worst case scenario take.)