It's difficult to say with precision what exactly is the truth of this claimed Tykocin community. There are hundreds of such individual communities for which similar allegations are made, some which seem to have a more or less credible narrative, and others with the usual absurdities encountered in camps like Auschwitz, Treblinka, etc. While it is true that mass executions took place by the Einsatzgruppen (sometimes entire small villages being wiped out, such as in reprisals in areas of heavy partisan activity), this all has to be understood in the context of WWII in defending an extended home front. Establishing secure areas often meant eradicating even the slightest threat of partisan activity or resources in an area (or engaging in reprisals to deter future activity). However harsh such measures might seem in hindsight (and from the comfort of our secure 21st century homes), Germany's enemies were serious threats and partisan activity found its shelter and recruitment among the Jews of Europe. Nuanced forms of justice is a luxury not affordable in such dire circumstances and civilians on both sides of the war (including but not limited to Jewish victims of Einsatzgruppen and the German victims of mass Allied bombing campaigns) perished in huge numbers as a result.
As for the people of Tykocin, however, much less is clear. A majority of the claimed evidence for the fate of these communities comes exclusively from witness statements which are rife with inconsistencies, exaggerations, embellishments, or outright fabrication. Virtually all of the claimed mass grave excavations come exclusively from Soviet sources (far from independent or objective) which have not even accompanying photographic evidence. German documents are extremely limited and most often have passed through Soviet hands. Testimony post-war is inconsistent and often appears contrived or coerced.
The bottom-line is that the evidence for what actually happened in any of these communities is sparse and problematic. One should factor these accounts into a broader understanding of "what actually happened" in the so-called 'Holocaust' however areas with more available research and claimed evidence (e.g. Birkenau, Treblinka, etc.) have become more central to this debate, given a greater baseline for understanding at these locations, enabling more precise questions and answers, which can shed more light on the question of the 'Holocaust' as a whole.
It is unsurprising no 'mass grave' has been excavated at Tykocin to corroborate the claim of 1,400 killed there. While perhaps not impossible in the context of Einsatzgruppen operations in some locations, the pattern of extreme exaggeration on both sides (Germans inflating their accomplishments in tackling partisan activity, Jews and Allies inflating their claimed suffering due to 'German barbarism'), warrants much skepticism as to what actually happened. Whether these Jews of Tykocin were actually deported by the Soviets prior to German arrival, dispersed or died from other causes, were captured or killed by the Germans, or perhaps even survived the war, and to what extent, remains unclear. And given that deeper investigation remains prohibited, it would be foolish to merely assume more than 1,000 murders based on unreliable claims, alone.