This question pertains purely to deadlock caused due to lock resources.
I am reading this article: Using a Clustered Index to Solve a SQL Server Deadlock Issue
They have explained how adding nonclustered index, or clustered index solves the deadlock problem.
The general idea is that - the UPDATE query will not block because of the index seek that will result in only few rows being locked.
However, the way SQL server works is - the engine at any point (for example after 5000 row level locks [Source: https://www.youtube.com/watch?v=EqfAPZGKifA at 30:25]) decide to elevate the lock to page or table level lock, thus locking the entire object (table for example). So is the solution given in this article - that adding clustered index is a solution to deadlocks - reliable?