You could certainly end up with a chain that's too short if you sized the chain based on only the small chainring and the small cog.
Please stop with the misinformation.
The small chainring and cog have no idea how big the large chainring and cog are.
Accidentally follow the procedure in that video with a short cage derailleur and an incompatible wide range cassette and crankset. Your drive train will be damaged the first time you shift into big-big.
It doesn't matter if you stay within the manufacturer's limits on derailleur capacity. Going outside of it can lead to damage, and it's called user error, whether by choice or ignorance.
Many, many times, and still doing it with no ill effects. I run it with my MTB, road, and cyclocross bikes. Single, double, and triple cranks.
- MTB: 7s 42/32/22 with 11-28 cassette & '94 Shimano Deroe LX & Shimano Deroe XT derailleurs
- Road: 7s & 11s using 53/39, 52/42, & 40/34 chainring with 12-24 (7s), 12-27 (7s), 11-25, 11-28, 11-30, & 11-34 cassettes and short cage (Shimano DA 7400 & 90s Campagnolo C-Record), medium cage (Shimano Ultegra 600, Ultegra 6800, Shinamo DA 9100, & SRAM Red eTap), and long cage (Sensah Empire Pro & Shimano Ultegra R800). My bike on the trainer was using a Shimano Ultegra 600 with an 11-25 & 11-28 11-speed cassette.
- Cyclocross: Sensah SRX Pro 1x with 36 & 40 front and 11-40 & 11-50 cassettes