I think both this point and OP's point can be true, depending on what the value is that people get from switching from Slack to a secure alternative. (And yes I've also been in teams that usually used Zoom or Meet but where the convenience of quick 1:1 calls in Slack was really nice!)
If a team is sufficiently desperate (as some we've talked to are) for something with end-to-end encryption but specific advanced moderation features that Signal lacks, then using Zoom, Meet, Signal, or some other video calling solution is fine. Sure it will be annoying, but there's a clear benefit the organization gets in return.
And yes, most teams do not get enough value from e2ee (or don't yet believe that they do) that giving up on the frictionlessness of a Slack huddle is worth it for them.
If we find product-market fit with an enthusastic niche of teams that need security, we'll be able to grow from there and address their needs for video calls too. If we can't find some users who are passionate about the particular advantages of what we're offering and willing to trade some features and convenience for those advantages, we're toast anyway :) (But we think we've found some!)
Slack's huddles are about eliminating the friction for hopping on a voice call. There's no waiting time for Zoom to start, no camera check, no "join with computer audio". You can go from a synchronous text-based conversation to a voice call in two seconds flat, as soon as it becomes obvious that's what is needed. That is the value proposition of Slack huddles, and external tools can't replace that.