A glide path lowers your equity percentage as you age; it changes the plan through two different channels, and they are worth keeping apart.
Channel 1: average risk
If the glide lowers your average equity over the horizon (starting at your current weight and stepping down), it reduces expected return. Over a long retirement, lower return means a lower ending balance and a higher chance of running short.
Channel 2: when the risk happens
If the glide holds your average equity constant (starting above your flat weight and ending below it), expected return is roughly unchanged, but more of your equity exposure lands in the early years, when the balance is largest and an early crash is hardest to recover from.
A glide can lower your probability of success either by earning less or by putting the risk in a worse place, even at the same average. The tier presets are average-preserving (their glide averages back to the flat weight), so picking flat vs. glide changes when you hold risk, not how much on average.
The equity share descends linearly between the start and end ages, the cash floor stays flat, and bonds absorb the remainder. Everything runs in today's dollars.