Throttling, as it has been applied in the wireless carrier industry, has generally been in reference to dropping to a set low speed essentially once some particular point and staying at that consistently until another particular and somewhat arbitrary point usually down the line (when a billing cycle is over, for example).
Deprioritization is something that happens only when connected to a particularly congested cell/tower and would apply only up to the duration of the congestion and only on that cell/tower. It also doesn't just lock to particular very low speed and nothing more that that, it simply prioritizes the connection lower than that of others who are not deprioritized, meaning that even when it's in effect the speeds can very well be quite fast, simply not at the full potential capacity that some others might be getting connected to that cell/tower.
So, just because people don't think there's a difference or don't care that there is one or don't want to acknowledge it because of some principle, or something else of that nature, it doesn't mean that there's no difference or that the difference doesn't really matter.