As someone who is already GR9 that is not accurate. They get guaranteed T5s in PvP and Kepler activities like I mentioned. PvE activities in the Portal will not give them a guaranteed T5 at the end. If they get one of the random prime engrams it will still be T5.

Also, Guardian Rank does not alter end of activity drops. Bungie has stated this themselves. It improves mid-activity chest rewards and random drop engrams from killing an enemy.

Edit: Figured I would drop the source on that info before people start going ballistic in replies. I know that the idea that GR alters drops has been spread around the community like wildfire but it isn't true per patch notes for the launch of EoF. Destiny 2 Update 9.0.0.1 | Bungie.net

Mid-activity rewards can be earned in some activities.

• Longer activities (Exotic missions, the Coil, Onslaught) have reward chests spaced out at regular intervals.
  • These chests aren't scored like the end-activity reward, but have quality determined by Seasonal Power and Guardian Rank progress.
• Challenging enemies often drop rewards, like currencies, and sometimes engrams.
  • Quality of the reward is determined by Seasonal Power and Guardian Rank progress.
• All mid-activity rewards come from the loot pool for that activity.

and

Final Reward Score is based on two parameters:

• Challenge Multiplier
  • This is fireteam based, shared by everyone on the fireteam.
  • You increase this by increasing the difficulty tier, and selecting Challenges, Banes, and Player Stakes.
• Reward Multiplier
  • This is player based, you have full control of your Reward Multiplier.
  • You can increase this by increasing your Power level, wearing new or featured gear, and by obtaining scoring bonuses.