That's not how the gap between sell-through and shipments works. If we estimate 1.5m Switch consoles on shelves now, then that's a pretty similar amount to how many Switch consoles were on shelves when entering the current fiscal year, therefore there is nothing to add onto the sell-through numbers for the current fiscal year. ~17.5m in sell-through also means ~17.5m in shipments.
Your maths suggest that Nintendo will annouce shipments of ~11.4m for fiscal Q3 (ended on December 31st), but VGC's sell-through estimates are actually pointing towards ~10.0m, so Nintendo will be at ~17m through three fiscal quarters, leaving them with 1m to go to reach 18m. Of course they'll revise their forecast upwards.
Should Nintendo announce notably higher shipments than ~10.0m, then VGC will adjust their sell-through estimates upwards. We'll know in a week because Nintendo's fiscal results are due on January 30th.