It's not actually Teksavvy's ATM lines.
DSL user -> incumbent/wholesaler's DSLAM (e.g. Bell) -> Bell's ATM/ethernet/mpls/whatever network -> demarc point -> Teksavvy's 3x GbE -> Teksavvy's network
The throttling is being done within Bell's network, the question that's unknown is whether they're intentionally throttling Teksavvy clients, somehow based on protocol, or if their network is simply overloaded and throttling everyone.
AFAIK, Teksavvy has an SLO for network performance, so it's quite possible Bell is breaking that completely. As Rocky mentioned, "wait for Tuesday" when they actually have a meeting with Bell and figure out what's happening.
Personally, I'm speculating that they're throttling based on a user's usage pattern (e.g. constant max rate) because their trunk lines are overloaded, and possibly prioritizing their business clients' traffic above others.