Notably, the body that created the fireball was initially only ~4 meters in diameter with a mass of 200,000 kg prior to atmospheric entry, traveling around 16 km/s (Brown et al. 2000). This is substantially smaller than the object that caused the airburst over Chelyabinsk (see Popova et al. 2015), which I'll discuss later. This mass was reduced prior to the explosion, as much of it was ablated as it traveled through the atmosphere.
I'm extremely skeptical of the claim because we have evidence that much of the area's telecommunications equipment functioned perfectly normally in the aftermath of the blast. This Master's paper relates accounts of the Tagish Lake event from locals in the area, including northern British Columbia and much of the Yukon Territory. It appears that immediately after the explosion, the radio station in Whitehorse was broadcasting as normal to a wide area:
The phone calls started immediately. With the contrail still bright, eyewitnesses called friends; they called the authorities; they called scientists; they called the media. Authorities and scientists and media all called each other, trying to nail down what had just happened. The phone lines at the local radio station of the Canadian Broadcasting Corporation filled with witnesses anxious to share their stories, and it was up to Peter Novak's calm baritone to mediate discussion on his daily program, The Valley Voice.
Whitehorse is claimed to have been near the blast (well, tens of kilometers below it, vertically). While landlines and individual cell phones would not necessarily be affected by an EMP, cell towers and other infrastructure might be - rendering communications difficult. This presumably includes equipment needed to broadcast the aforementioned radio show. It would be a bit surprising if there was no effect on telecommunications nearby if the electrical grid had severe problems - and yet folks seem to have been able to communicate just fine, including in Whitehorse.
It's not implausible that a meteor could cause electromagnetic activity in the atmosphere, but there's a significant difference between electromagnetic activity and an electromagnetic burst. There are a couple known cases of electrophonic bursters (which have an unfortunately similar name), meteors which were accompanied by pops and clicks. It has been proposed (Beech & Foschini 1999) that the sounds are caused by shock waves propagating through the plasma caused by the ablation of the meteor as it travels through the atmosphere, causing pulses in the electric field of the plasma and in turn causing sounds, which can propagate through the atmosphere.
These pulses are distinct from EMPs in the typical sense, of course. Remember the Chelyabinsk burst I mentioned? It did cause electrophonic sounds, but it did not cause an EMP. It did affect telecommunications, but through shock waves, not an electromagnetic pulse (Popova et al. 2013):
Electrophonic sounds were heard (SM Sect. 1.6), but there was no evidence of an Electromagnetic Pulse (EMP) under the track in neighboring Emanzhelinka. Due to shock wave induced vibrations, electricity and cell phone connectivity was briefly halted in the Kunashaksky district at the far northern end of the damage area.
The shock wave from the explosion did damage - not any sort of electromagnetic activity. Moreover, the Chelyabinsk object was substantially more massive than the body that impacted at Tagish Lake. The above paper estimates a mass of about 13,000,000 kg and a diameter of around 20 meters, traveling at 19 km/s.
