Coast Starlight service started on AmDay, 5/1/71. On that day, the former Coast Daylight was started going via Oakland and 3 days a week it ran through to Seattle (with a short lived through sleeper to San Diego). It was re-routed via Oakland so it COULD run through to Seattle. You are thinking about the SP trains running immediately prior to Amtrak, the Cascade (11/12) running Oakland-Portland 3 times a week and the Coast Daylight (98/99) running daily Los Angeles-San Francisco. Amtrak never ran two separate trains LA-Oakland and Oakland-Seattle. It was a through train at the outset, the first single train through service from LA to the Pacific Northwest since SP's "West Coast" (LA-Portland via the San Joaquin Valley) ceased service north of Sacramento which happened fairly early on (late 40s/early 50s?). Amtrak touted the restoration of through LA-Pacific Northwest service as an example of "We're Making the Trains Worth Traveling Again!"
At first it was called the Coast Daylight/Starlight. When the Oakland-Seattle part of the run went daily, they dropped the "Daylight".
As I mentioned before, at first it used SP's Coast Daylight and Cascade train NUMBERS (98-11, 99-12), but it was always one train under Amtrak on the days it went through. The origin of the Amtrak numbers 11(12)/(13)14 was as I described, to accommodate the 2 numbers SP's numbering system required to be used for a through LA-Portland-Seattle train. Not sure how BN numbered it north of Portland, 11 and 14 or something else like 1011 and 1014 (which for some reason popped into my head, maybe some dim memory of it).
So there is a kernel of truth, there was no through train LA-Portland/Seattle, but that was prior to Amtrak. Also, the original Coast Daylight/Starlight only ran through to Seattle 3 times a week while it was daily LA-Oakland. But there was never a connection required under Amtrak. And the LA-Portland connection under SP was actually at Martinez with the San Joaquin Daylight, not the Coast Daylight.
PS-this is not all purely academic information. I rode the Coast Daylight/Starlight in Amtrak's early days, when everything was SP from the tomato stripe cars, to the conductor's uniforms, to the ticket stock. About the only non-SP thing about it was it turned and serviced in Los Angeles at Santa Fe's 8th Street Coach Yard. Oh, yeah, and on the through days it had a dining car, which was unlike late-stage SP service.